Facebook is reported to have confirmed that it does indeed use the mobile phone number that users give it for 2-Factor Authentication (2FA), for ad-targeting. This comes after an investigation revealed that Facebook also uses the contact details you give it, but don't make public, for ad-targeting.
Facebook has always used all the information a user provides it for ad-targeting but this was generally information that a user added to their profile and agreed for it to be used in searches and the targeting of advertisements. In these cases, the social media platform has taken the liberty of using information that the user didn't consent to being used for ad-targeting.
“We use the information people provide to offer a better, more personalized experience on Facebook, including ads. We are clear about how we use the information we collect, including the contact information that people upload or add to their own accounts. You can manage and delete the contact information you’ve uploaded at any time," said a Facebook spokesperson.
In the first example, Gizmodo did an investigation which ended with one of their writers being able to target an ad using another users hidden telephone number, thus having the ad only displayed for that user. This, without a dount, is unethical. Not only is the use of information that users didn't consent to worrying, but this is being discovered not so long after Facebook said it is correcting mistakes made after the Cambridge Analytica saga.
The second example is, to put it mildly, dodgy and despicable. The mobile number you give Facebook for 2FA is supposed to be strictly used to send you a 2FA code to complete your log in. Yet, somehow, a group of people at Facebook thought it would be great to harvest these numbers and effectively sell them to marketers for them to target ads.
When you throw in the recently publicized security flaw that is said to affect "about 50 million Facebook users" (a rather not so specific number that sounds more like a guess than someone knowing what's going on - or they are hiding the real number?) you start to question what "really" is going on at Facebook and whether or not the platform has become too big for the team to manage.