WhatsApp will do what it once said would never happen: let businesses use the messaging app to serve ads to users.

In a move that was inevitable once it was acquired by Facebook for $22 billion in 2014, WhatsApp has put its users on notice that it will soon begin sharing their phone numbers, and selected other data, with its parent company.

This information will then be used to offer customers "more relevant" Facebook ads, new "ways for people to communicate with businesses" via the app, and new friend suggestions, the blurb reads.

WhatsApp said on Thursday that "by coordinating more with Facebook, we'll be able to do things like track basic metrics about how often people use our services and better fight spam."

The change seems set in stone for new sign-ups, but existing users will apparently have the chance to opt out of the chance to "improve your Facebook ads and products experiences." Once the updated terms and privacy policy have been accepted in-app, users have 30 days to opt out. WhatsApp's full new terms and changes are available here.

While hardly surprising, it is a massive U-turn to WhatsApp's original ad-free business strategy, which carried a nominal yearly subscription fee of 69p ($1). It was dropped for good in January, leading analysts to surmise that changes would be coming—even as WhatsApp denied any plans for an ad-wrapped future.

For years, WhatsApp's founder Jan Koum insisted that it wouldn't be used to harvest data for advertisers. In 2012, he said, "these days companies know literally everything about you, your friends, your interests, and they use it all to sell ads."

When we sat down to start our own thing together three years ago we wanted to make something that wasn't just another ad clearinghouse. 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.

Two years later, in a blog post marking the Facebook acquisition, WhatsApp insisted "nothing" would change for its millions of users. This attitude has clearly not survived contact with the free content ad network's remorseless attitude to hoovering up data. WhatsApp said it was now part of "the Facebook family of companies,” implying that user data might also be shared across Instagram and Oculus Rift.

However, WhatsApp insisted that it won't allow third-party banner ads on the app, and claimed its end-to-end encryption means it can't read or store users' messages, nor share them with third parties.

In 2012, Koum wrote "no-one wakes up excited to see more advertising."

Advertising isn't just the disruption of aesthetics, the insults to your intelligence and the interruption of your train of thought. At every company that sells ads, a significant portion of their engineering team spends their day tuning data mining, writing better code to collect all your personal data, upgrading the servers that hold all the data and making sure it's all being logged and collated and sliced and packaged and shipped out... And at the end of the day the result of it all is a slightly different advertising banner in your browser or on your mobile screen. Remember, when advertising is involved you the user are the product.

Critics of WhatsApp's sudden, if inevitable, reverse ferret are piling in to attack the move, however.

"Facebook promising to protect the privacy of WhatsApp users is like the fox promising to protect the rights of the chickens in the coop. It was clear from the beginning that this would happen," privacy campaigner Aral Balkan told Ars. "The saddest part of all this is that we are, once again, shocked to find that Facebook has acted entirely according to its core business model."

He added: "If we continue to delude ourselves that companies like Facebook and Google are somehow forces for good in the world instead of factory farms for human beings—and unless we start regulating them as such—we are going to have our human rights eroded one policy update at a time, like slowly boiling frogs."

Meanwhile, the UK's data watchdog said it was aware of concerns relating to WhatsApp's shift in strategy to allow ads onto the service. Information commissioner Elizabeth Denham said: