When mobile messaging service WhatsApp was bought for $19 billion (€17 billion) by Facebook in 2014 , it promised there would be no significant change to its terms of use.

Now WhatsApp, which has more than one billion users worldwide, has announced plans to share users’ phone numbers with its parent company, updating its terms and privacy policy for the first time in four years to reflect the change.

Users will have 30 days to decide whether to opt out of their information being used for ad targeting on Facebook, but they will not be able to opt out of their data being shared with the social network.

Ad targeting based on this data will take place on Facebook, which has 1.71 billion monthly active users, with WhatsApp saying it will not place ads on its own platform.

It will, though, permit business accounts offering customers the sort of information – airline updates, banking transactions – currently available via SMS or other services.

Privacy

WhatsApp said that co-ordinating with Facebook would allow it to improve its service, and that its end-to-end encryption of messages would mean user privacy would be maintained.

“Even as we co-ordinate more with Facebook in the months ahead, your encrypted messages stay private and no one else can read them. Not WhatsApp, not Facebook, nor anyone else,” it said yesterday.

Despite these assurances, many users will probably be unhappy with the move.

The app’s popularity has rested on its uncluttered, ad-free and (unprofitable) business model, which gives users an easy way to communicate without having their personal data exploited.

Asked to comment on the change, a spokesperson for the Data Protection Commissioner said: "This office engages regularly with the technology multinational data controllers it regulates which are based in Ireland, including Facebook, and accordingly, monitors relevant developments on an ongoing basis.

“It should be noted that WhatsApp is a US-based entity.”