WhatsApp users just got a taste of the old dictum, "If you're not paying for the product, you are the product." And not everyone liked it.
The messaging giant announced on Thursday that it would share its huge database of users' mobile numbers with Facebook. The social media company announced in 2014 that it was buying WhatsApp for $19 billion, a deal that later grew to $22 billion as the value of Facebook's stock rose.
According to WhatsApp, the move was to help its parent company show Facebook users more relevant advertisements and friend recommendations.
But some WhatsApp users - the service has more than a billion of them - accused the app of breaking its long-time privacy promises.
Back in June 2012, WhatsApp wrote a blog post explaining that it didn't sell adds because "no one wakes up excited to see more advertising, no one goes to sleep thinking about the ads they'll see tomorrow."
After they were bought by Facebook, WhatsApp sought to reassure users that "absolutely no ads [would be] interrupting your communication" - a theme it continued on Thursday when it said that the company's "belief in the value of private communications is unshakeable."
That's not how some users saw it, however.
Nick Savvides, a "security evangelist" at Symantec, said the level of upset reflected the fact that privacy was now a big issue for all social media users.
"One of the major reasons why data privacy is becoming such a concern is because there is now a clear understanding among consumers that their data holds value,"he added.
In February WhatsApp dropped its $1 token charge and acknowledged that it was looking at the possibility of making money from finding ways for users to"communicate with businesses and organisations that you want to hear from."
It's a point that some users acknowledged on Friday.
Several tweets highlighted that it was, in fact, possible for WhatsApp users to opt out of sharing their mobile numbers with Facebook.