Facebook tells users that giving the company their mobile phone number will help keep their account secure. Until a few weeks ago, however, the social network’s self-service ad-targeting tools could be massaged into revealing a Facebook user’s cellphone number from their email address. The same flaw made it possible to collect phone numbers for Facebook users who had visited a particular webpage.

Oh wow. This is why I was very reluctant to give platforms access to my phone number, and still have a bad feeling for the few where I enabled 2fa with a phone number. Lets face it: No matter how good the intention (and in case of the data mining silos like Facebook, Google and Twitter I have my doubts) or the technology, even if they promise to use your private data for internal or security reasons only, you have to expect that this data may become public in one way or the other. So consequently: Never give platforms access to data that you wouldn't publish publicly.