Isn't this like Whisper 2.0?
Doesn't matter how anonymous I am to other Facebook users, Facebook will know who I am and will undoubtedly be collecting what I say and knowing I said it.
If I anonymously revealed in confidence to anonymous friends that I'm into hardcore S&M, I'll bet I soon start to see Facebook ads for whips and "suggested posts" regarding a company that helps you build your own dungeon. Facebook wouldn't add something like this if they didn't see it as a way to further monetize you - now that they know everything public about their users, they want to know their secrets. They won't tell Facebook voluntarily, but they'll tell their friends, especially if they feel they have some level of anonymity, but doing so in this way will also be telling Facebook.
Who's to say who Facebook might share this information with, deliberately or not? They were, after all, one of the companies who were voluntarily cooperating with the NSA spying. They also make all their money off selling personal information (i.e. as with Google, we are not their customers, we are their product) and to say Facebook has a spotty record on privacy is a huge understatement. At least Google pretends like they're sorry when they get caught with their hand in the cookie jar. Facebook acts like the customer is in the wrong for calling them out on their abuses!