As the Craig in Craigslist, the free online noticeboard that changed everything, Craig Newmark can surely get his hands on just about anything. His new home in Greenwich Village, New York, contains everything from an ancient Roman mosaic to 18th-century British portraits to Simpsons figurines to artworks by his beloved Leonard Cohen. But something is missing. Something vital.

“We’re low on bird seed now,” Newmark observes anxiously. “That’s a crisis.”

The scale and scope of the crisis become evident when you understand Newmark’s ornithological obsession. During an hour-long conversation, his eye keeps wandering to the small garden where mourning doves, house sparrows, cardinals, blue jays and “a hopefully limited number of pigeons” come and go. Just last night he installed a webcam so he can watch them all remotely. For good measure, there are numerous photos of birds on the walls and a papier-mache model, made by his 11-year-old nephew.

“I love birds for reasons unknown,” Newmark says. “We’re observing that the doves are not that nice to each other and we also see them fighting with the sparrows. The sparrows are much smaller, but the more aggressive sparrows can chase off a much larger dove. So I’ve named them Cersei and Daenerys.”

It may be that Newmark feels more comfortable around feathered friends than the human variety. He is a self-declared “nerd of the old school, 1950s style”, squirrel watcher and sci-fi fan who, sitting in a jacket, trousers and slippers, cheerfully admits he is “simulating” social skills. He is a computer geek who checks email obsessively and in 1995 founded Craigslist – which is, with about 50bn page views a month, one of the world’s most popular websites. Now 66, he is a survivor of the age of internet idealism, before fake news and perpetual outrage cast long shadows.

But he is also a divisive figure. Some express gratitude to Craigslist for life-changing opportunities to find a spouse or a job. Others condemn it for gutting the classified advertising market and accelerating the demise of local newspapers. This ambivalence was captured by a New York Times headline last October that described Newmark as a “newspaper villain” and another last month that called him a “new friend to journalism”. The latter referred to Newmark’s latest act of philanthropy, a $6m (£4.8m) gift to Consumer Reports – the biggest donation in the 83-year history of the nonprofit watchdog – that will be used to set up a digital division to scrutinise products and platforms, including social media.

“I’m mostly concerned about the way social media platforms can be weaponised, that they sometimes forget to provide informed consent regarding the uses of your personal data,” he explains. “I do feel that any site should tell you what it would like to collect and what it would like to share with others and then ask your permission.”

The political consultancy Cambridge Analytica, which harvested the data of up to 87 million Facebook users during the 2016 US presidential election, is one such example, he says. However, Newmark, who has met Facebook’s founder, Mark Zuckerberg, but does not know him well, declines to criticise him or suggest Zuckerberg has created a monster.

“I have no idea what he’s done personally. I focus on how we all work on this together. It’s all hands on deck. People need to enlist much like in the US after the attack [on Pearl Harbor] in December of 1941, much like people during the Battle of Britain. There are foreign adversaries who’ve come out, published their public statements and say that they’re at war with us.”

Victory is unlikely to be as clear as in the second world war, however. “The measure of success would be that I would be able to choose from a high number of publications wherein I know that I can trust everything they say with the occasional mistake, which they will then fix. Because news is hard; you’re going to make mistakes, then you correct them and Bob’s your uncle.”

Newmark, who has previously observed “a trustworthy press is the immune system of democracy”, prefers not to use the term “fake news” – perhaps it has been tainted for ever by Donald Trump. He reasons: “Some people have said fake news is news that I don’t like, so I will talk about misinformation or disinformation and that is often either false news or false witness, either weaponised information or just carelessness.

Craigslist ... classified information. Illustration: Guardian Design

“If it’s carelessness, you can tell someone: ‘Hey, you got it wrong; here’s the evidence.’ If it’s disinformation, they don’t care, they will just publish it again. Meanwhile, when you point out the problem, they might decide that you’re a target for harassment.”

There is an entire ecosystem at work, he continues, that can enable a falsehood from the obscure reaches of the web to jump on to millions of TV screens with dizzying speed. “It’s a small amount of disinformation originating in some of the social media platforms used by foreign adversaries and their domestic allies. They get amplified: there’s multiple levels including conspiracy sites, then news sites which don’t care about fact-checking. And then once that becomes news, sometimes that emerges into conventional or mainstream media.”

How can it be stopped? “People in mainstream media can just do things like fact-checking. They could avoid giving airtime or space to people they know routinely disinform and they can just avoid amplifying disinformation.” The new journalism centres he is funding are considering whether journalistic ethics codes should explicitly say: “Don’t amplify disinformation.”

If this sounds like a swipe at Rupert Murdoch’s Fox News, Newmark ducks an invitation to be critical. He is similarly circumspect when asked if tech giants should be broken up, if the existence of billionaires is unethical (he is a mere millionaire) or whether the decline of local journalism contributed to the ascent of Trump. “I don’t know,” is his default response.

Speaking of the president, Newmark, who has donated to Democratic presidential hopefuls Kamala Harris and Elizabeth Warren, gives an unorthodox answer when asked what he thinks of his term in office so far. “He’s been very effective in terms of getting elected.” He pauses. “I am very passionate about understatement. I know the British practise that. I learned little by little. For example, I spent some time in Edinburgh last year and I now really like haggis. I’ve also acquired a taste for Scotch whisky. I understand the English do not drink very much. I am still a very light drinker, but I want to not exceed the delicate sensibilities.”

Newmark embraces his nerdiness. After university, he worked for the computer giant IBM. He also took ballet and jazz dance classes in an attempt to meet women, but suffered a hernia and was hospitalised. Aged 40, he took a programming job with the financial firm Charles Schwab and moved from Pittsburgh to San Francisco, then switched to more lucrative freelance work.

In 1995, he began sending out an email list of events to a dozen friends. As word spread, the group expanded. Before long, the list included job vacancies and places to rent. Soon, Craigslist ruled the world. Looking back now, would he have done anything differently?

“In that time frame, probably not because I got lucky. For example, when the mailing list I was running needed to have a name – I’m very literal as a nerd; I was gonna call it San Francisco Events – people around me told me they already called it Craigslist. I had inadvertently created a brand.”

But in a portent of the wider web’s dark side, it has been abused by criminals. In 2007, for example, a woman from Minneapolis pleaded guilty to using its personal ads to run an underage prostitution ring. Last year, Craigslist took down its personals section after Congress passed the Fight Online Sex Trafficking Act, designed to crack down on sex trafficking of children, which could have made the site liable to prosecution when its users broke the law.

Newmark could have sold Craigslist for a huge fortune and joined the Silicon Valley super-elite, yet never considered making a quick profit; the site itself remains defiantly basic and unadorned by banner ads. While his home in New York, a former stable dating back to 1848, is certainly swanky and he has another place in San Francisco, Newmark no longer owns a car but prefers to travel in taxis and on public transport.

“When I had to make Craigslist from a hobby into a real company, I decided, given what I learned in Sunday school, I should monetise it minimally. The business model being: doing well by doing good. That’s a lesson from [his teachers] Mr and Mrs Levin in Sunday school: you should know when enough is enough. In more current terms, it’s no one needs a billion dollars. Also, in very pragmatic terms, take less and give more, so I’m in the process of doing that with forthcoming announcements.”

Newmark at his home in Greenwich Village. Photograph: Caroll Taveras/The Guardian

Newmark, who married Eileen Whelpley in 2012, adds: “The deal is I live comfortably. I’m helping my family and some friends live comfortably. That ain’t bad. I can afford to have a garden out there with birds. Children are not a consideration, but we do have 21 nephews and nieces; No 22 is under construction. We’re looking forward to their launch party.”

And yet there is a persistent, widespread view that Newmark’s humble-looking site destroyed classified advertising, one of the newspaper industry’s most valuable cash cows. Last October’s New York Times article – the one that called him a “villain” – stated: “Researchers eventually estimated that Craigslist had drained $5bn from American newspapers over a seven-year period. In the Bay Area, the media was especially hard hit.”

The decline is continuing and even accelerating. The US media is reportedly facing its worst job losses in a decade with about 3,000 people laid off or offered redundancy in the first five months of this year. Last month, the Vindicator, the sole daily newspaper in Youngstown, Ohio, announced that it was closing after 150 years. “News deserts” are spreading across the US with alarming implications for democracy.

Newmark says: “I feel very strongly about the issue because journalists lose jobs and that’s accelerated this year. But in the last two or three years, I found economists and industry analysts [who have looked] at the numbers – they adjust for inflation, household growth and all that – and they say that there’s two things that happened in the early 50s, one of which was TV news, and from there [we have seen] a straight-line decline in newspaper circulation revenues, accelerated 10 years ago by the advent of the platforms.

“Now, my gut tells me that Craigslist must have had some effect, but apparently it’s pretty tiny. Mostly it’s just the effect of TV news. Now the other event that happened in the early 50s was my birth but, barring any mention of the book of Revelation, I don’t think I’ve had any effect of any sort.”

So does he think he is unfairly demonised by some journalists and does that rankle? “I can’t fault them for not getting around to doing the fact-checking. You know people are busy and a lot of people in journalism have been told by their managers that fact-checking may not be critical.”

When the City University of New York (CUNY) named its graduate school of journalism after Newmark following a $20m donation, there was a backlash. Felix Salmon, the chief financial correspondent at the Axios website, tweeted: “It’s utterly bizarre to name a journalism school after the man who almost single-handedly destroyed local newspapers.”

How does that make Newmark feel? “I do wonder why he hasn’t talked to the economists or industry analysts involved,” he says, android-like, before pivoting to a paean for CUNY’s work in giving a break to people from different ethnic, religious and economic backgrounds and boosting diversity in newsrooms.

Thus he firmly rejects any notion that all the philanthropy – an estimated $50m in the past year including to New York Public Radio, new publication the Markup and local journalism efforts such as the American Journalism Project – is an attempt to assuage guilt, a reach for atonement. “That takes an active imagination that I don’t understand. I have very little imagination.”

The ascent of Craigslist, Wikipedia and others also led some to predict that user-generated content would conquer all, providing the ultimate expression of grassroots democracy. To the storied newsrooms of the US, it was a naive and dangerous fantasy.

Newmark in 2006. Photograph: Justin Sullivan/Getty Images

Newmark, by his own admission not a journalist, says: “I had great hopes for citizen journalism 10, 15 years ago. It hasn’t worked out. One reason is that journalism is a profession. You have to know how to write well. You have to fact-check. You have to know how to develop sources, often over years. You have to have specialised knowledge on a beat like disinformation or crime or birds. Citizen journalists can complement what’s going on and, sometimes, citizens come to journalism with skills.

“I was way over-optimistic because I didn’t realise the need for professionalism in the manner that I talked about. Now I think more: what are the practical problems of professional journalism? For example, we’ve seen a couple of cases where bad actors will try to really hurt a publication by engaging in lengthy, frivolous lawsuits. There is a great need for shared risk pool insurance, media insurance in the US, and I talk to people about that.”

Newmark denies, however, that he was pollyannaish about the web itself, even after the much-publicised bile, poison and rancour of the 2016 presidential election and what some now call a cold civil war in the US. Social media fights, he insists, get attention but are not representative of what is really going on. Much of it is manufactured.

“Americans are much more reasonable and moderate than what you might guess when you see a little Twitter war. But I’m guessing that the purpose of many Twitter wars is to polarise people and, in fact, we’ve seen that happen because you can often trace some of the fighting groups to the same location. Outrage is profitable. Most of the outrage I’ve seen in the online world – I would guess 80% – someone’s faking it for profit.”

After 24 years at Craigslist, often dealing with customers directly, Newmark reflects: “When you’re looking at tens or hundreds of ads in a day you get a better grasp of what people are really like than the more dramatic flare-ups. So that’s the basis for my optimism.”

Indeed, he remains convinced that the internet is still a positive for humanity. “It allows people of goodwill to get together and work together for common good. Bad actors are much louder, they make for more sensational news and we’re seeing a period of that now. The US, in a way, is lucky. Bad actors interfering with our elections may have had some success but their success is not complete and it means that people of goodwill are fighting back vigorously.

“I play a microscopic role in that. I find the people who do the real work and then I help fund them, I get them to talk with each other, and I’m funnier than they are. I remind myself that a nerd’s got to do what a nerd’s got to do and that’s my driving slogan. In my gut, that incorporates the notions of treating people like you want to be treated.”

How, then, would he like to be remembered? He considers for a moment before stepping outside to see the birds. “As a nerd that stayed true to his nerditude,” he says. “And that I knew that I wasn’t as funny as I think I am.”

• This article was amended on 15 and 16 July 2019. An earlier version misnamed mourning doves as “morning doves”. In addition, it misquoted Craig Newmark as saying “… don’t amplify this information”, when he said “don’t amplify disinformation”. This has been corrected.