Level 36
Facebook exchanges your privacy for ad revenue
As discovered by Gizmodo's Kashmir Hill, Facebook builds a complex advertising profile for every one of its users containing everything from their two-factor authentication phone number to information gathered from other people's profiles.

Phone numbers handed over by users to Facebook for two-factor authentication are being used by the social network to add an extra targeting vector for advertisers using its platform.
This has been discovered by a research team after phone numbers they used for test accounts were being targeted by advertisers in about a couple of weeks.
Instead of using two-factor authentication phone numbers as an extra security measure, Facebook added them to their ad-targeting profiles
Moreover, she also recommended to users who are bothered about this practice to use an e-mail address for enabling two-factor authentication for their account.

The weird part in all of this is that Facebook decided to allow their users to use e-mails for two-factor authentication about four months ago, in a blog post by Scott Dickens.

"In describing this work to colleagues, many computer scientists were surprised by this and were even more surprised to learn that not only Facebook, but also Google, Pinterest, and Twitter all offer related services," Alan Mislove one of the researchers said. "Thus, we think there is a significant need to educate users about how exactly targeted advertising on such platforms works today."