In 2015 Twitter lost one chief executive, gained another, and nearly tossed them too, before eventually settling down with him – incidentally, one of the men who had founded the company way back in 2006.

It also finally admitted that it sucks at dealing with abuse on the site, made its first tentative steps from “platform” to “publisher”, and began testing the most controversial new feature it’s ever introduced.

Oh, and its stock price fell by one-third over the year, and is now two-thirds where it was at its peak.

2015 for Twitter has been … difficult.

The year began at a low point, with the gamergate movement, formed in the latter half of 2014, evolving from its roots as a focused campaign against the influence of “social justice warriors” (in the form of left-leaning critics and creators) in the world of gaming. By 2015, it had spun into a more general-purpose anti-feminist group, hovering around the same milieu of reactionary online movements as mens’ rights activists and the “dark enlightenment”.

That played out as an explosion in both the quantity and visibility of online harassment – something Twitter was uniquely ill-equipped to deal with. The service is built on the conception of a public timeline, with any account being able to contact any other. It also eschews Facebook’s strong linkage to real names and identities, in favour of helping those who want to tweet pseudonymously or on behalf of others. That made it tricky to deal with abusive tweeters by simply blocking them from the site, since they could just re-register under a different name.

It wasn’t just gamergate. The number of people hit by abuse on the social network seemed to be growing incessantly: Rebecca Adlington revealed that abuse on Twitter had harmed her self-confidence in the wake of the London Olympics; Chloe Madeley, daughter of TV presenters Richard and Judy, was targeted for harassment by people upset about her mother’s comments about convicted rapist Ched Evans; and Guardian columnist Lindy West was so deluged by online attacks that she wound up confronting one of her trolls, who had assumed her dead father’s identity to abuse her.

That last story led, in February, to Twitter boss Dick Costolo telling staff that the company needed to shape up. “Twitter sucks at dealing with abuse and trolls on the platform, and we’ve sucked at it for years,” he wrote in an internal memo, promptly leaked to the Verge news site.

“I’m frankly ashamed of how poorly we’ve dealt with this issue during my tenure as CEO,“ Costolo wrote. “It’s absurd. There’s no excuse for it. I take full responsibility for not being more aggressive on this front. It’s nobody else’s fault but mine, and it’s embarrassing.”

But admitting you have a problem is only the first step to recovery. Tackling abuse on a platform like Twitter is hard: it either requires automated systems to be able to discern the difference between a joke and a threat, and between friendly ribbing and sinister harassment, or it requires an exorbitant outlay on human moderation so that people can make the tricky decisions.

Twitter tried to do both, boosting its moderation team to better respond to reports of abuse, while also introducing new features that, it hoped, would protect users. In March, Costolo launched the “quality filter”, an optional tool for verified users (who often see the bulk of abuse on the platform) to eradicate abuse from their notifications. The abusers themselves weren’t touched, but their targets didn’t have to see what they were saying any more.

In April, the company went a step further, turning on a weaker version of the quality filter for all users. It couldn’t be turned off, a choice that earned the company some criticism from those worried that they would miss important notifications flagged as a false positive by the abuse filter. But many welcomed the change. Zoe Quinn, who had been at the head of gamergate’s enemies list, said that it was “good to see them actively stepping up”.

But all the attention on harassment in the first half of the year distracted from another problem which would come back to bite Costolo hard: Twitter’s user growth had stalled.

Twitter is big, boasting 320 million monthly active users, but it’s also a publicly traded company, with shareholders to please and an incessant pressure for growth. It is also very clearly number two in its field, hovering around a third of the size of Facebook, a company so big that its growth potential is limited by the population of the earth.

As a result, there had been murmurs of a shareholder rebellion for long enough that when Costolo eventually announced he was stepping down in June of this year, the only real surprise was his insistence that his departure was his own decision.

Stepping in as interim chief executive was Twitter’s first CEO and co-founder Jack Dorsey, who had been the board’s executive chairman since 2011. It should have been a triumphant return, but Dorsey’s long-term position at Twitter was doubtful, since he was already the chief executive of Square, the payment processing company he had set up after his first departure from the position of CEO.

As a result, Twitter wondered through a long search for Costolo’s successor, repeatedly dropping hints that Dorsey would need to quit Square to qualify for the top job. And yet, in October, he was confirmed as the new chief executive away. He tweeted the news – naturally – pointedly adding that “I’ve been CEO of both companies for over three months now.”

But even as the triumphant boss returning to the company he founded, Dorsey still faced the same problem as Costolo. Any social network is a tricky proposition for new users, who have yet to build up the connections that make the site hum, but Twitter is particularly awful to make a fresh account on: the best accounts to follow are hard to find, and often hard to understand to new users. On top of that, the chronological feed means that it’s hard to read every post, and easy to feel like you’ve missed out if you don’t. And so, unsurprisingly, new users shy away.

Just days after Costolo announced his departure, Twitter revealed its biggest attempt to fix that issue to date: Project Lightning, soon renamed “Moments”. It sees Twitter reinventing itself as a publisher, hiring editorial teams in multiple continents to comb the site for good content and collate it all together in readable, shareable packages.

It aims to answer the question “how can I find out what’s happening right now on Twitter”, says Jo Geary, the project’s head in Europe. She cites the example of Tim Peake, the second-ever British astronaut, who travelled to the ISS in December. “It might not be so easy … for someone who’s new to Twitter to find the best accounts covering the launch.”

Importantly, Moments can also be seen by logged-out users, and those who don’t have an account at all, something which – if it catches on – could be the “onboarding” experience the service has sorely needed.

But Moments is a Costolo project, even if it only hit the public under Dorsey. The really radical overhaul needed the return of the founding father to even be considered: the death of the timeline.

An increasing number of users have seen their accounts become a testbed for a new, Facebook-esque feed, presenting tweets out-of-order. The new feed is presumably, but not explicitly, ordered so that the most interesting tweets are shown, regardless of when they were posted.

For existing users, and those who focus on the site’s advantage when it comes to coverage of live events, the change is near-heresy, and while it’s been mooted for over a year, it could only have moved from proposal to reality under someone with the personal authority of Dorsey.

As for whether it will graduate out of testing, and whether it can save Twitter – well, that’s a question for 2016.