When WhatsApp, the messaging app, launched in 2009, it struck me as one of the most interesting innovations I’d seen in ages – for two reasons. The first was that it seemed beautifully designed from the outset: it was clean, minimalist and efficient; and, secondly, it had a business model that did not depend on advertising. Instead, users got a year free, after which they paid a modest annual subscription.

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Better still, the co-founder Jan Koum, seemed to have a very healthy aversion to the surveillance capitalism that underpins the vast revenues of Google, Facebook and co, in which they extract users’ personal data without paying for it, and then refine and sell it to advertisers. In a blog post headed “Why We Don’t Sell Ads” written in June 2012, for example, Koum quoted approvingly a memorable line uttered by Tyler Durden (played by Brad Pitt) in the movie Fight Club: “Advertising has us chasing cars and clothes, working jobs we hate so we can buy shit we don’t need.”

“When we sat down to start our own thing together three years ago,” Koum wrote, “we wanted to make something that wasn’t just another ad clearing house. We wanted to spend our time building a service people wanted to use because it worked and saved them money and made their lives better in a small way. We knew that we could charge people directly if we could do all those things. We knew we could do what most people aim to do every day: avoid ads.”

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And Koum was as good as his word. WhatsApp grew and grew, because it did what it said on the tin. Its growth was funded by $58m from Sequoia Capital, the only venture capital firm to invest in it. By February 2013, WhatsApp claimed 200m active users worldwide, and had a valuation of $1.5bn – not huge by Silicon Valley standards but pretty good for an outfit that had an honest business model.

And then, in February 2014, something strange happened. Facebook offered to buy the company – for $19bn – and Koum and co took the bait. Given that Facebook’s business depends on selling ads, most of us wondered what had happened to Koum’s admirable principles and concluded, gloomily, that everyone has a price.

But for a while WhatsApp continued as before within the Facebook stockade. Not only did it not sell ads, but in November 2014 it announced that it was introducing end-to-end encryption for all WhatsApp communications – which meant that nobody, not even Facebook, could read (and thereby monetise) its users’ messages. So the puzzle continued: why on earth had Mark Zuckerberg paid such a whopping price for a service that he couldn’t exploit?

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Now we know. On 25 August WhatsApp announced that it was changing its terms and conditions and its privacy policy. In a blog post which is a masterpiece of lawyerly euphemism, it tells us that it is going to “share” with Facebook its users’ phone numbers and details of the last time they signed on to WhatsApp. Every user is asked to click “Agree” to this proposition – although, of course, they can always reverse this agreement if they can find the relevant section of their settings.

Needless to say, this radical change has nothing to do with the needs of WhatsApp’s corporate owners. Perish the thought: it’s to improve things for you, the user. It’s all about using “your WhatsApp account information to improve your Facebook ads and products experience”. And, just to make sure you understand the magnitude of the decision you are about to make, “If you tap ‘Don’t Share’, you won’t be able change this in the future”.

Seasoned observers of the computer industry will recognise this for what it is: just another illustration of the power of the default setting. In marketing-speak, it’s how to “Nudge Your Customers Toward Better Choices” – an implementation of the philosophy set out by Cass Sunstein and Richard Thaler in their book Nudge: Improving Decisions About Health, Wealth and Happiness.

Of course, it’s true – as Sunstein and Thaler argue – that defaults can sometimes be used to ensure that people do things that are good for them, like ensuring that they enrol in a pension scheme. But in the computer industry, defaults are often deployed purely in the interests of corporations.

So if you’re a WhatsApp user, don’t fall for this particular wheeze: go to “Settings”, select “Account”, “Share my account info” and tap on “Don’t Share”. And do it now, because time’s running out.