My biggest fear about FB isn't regarding privacy, but its enormous ability to spread misinformation.
And of course, the more you associate and interact with groups that fit your opinions and interests, the more Facebook and other ad-supported services will try to prioritize showing you those posts, so the echo chamber effect is amplified.
And when you step out into the real world, you'll be inclined to think of others that don't share your "informed" opinion as uninformed or naive, after all you have seen all this evidence to back you your opinions and theories. [Edit:spelling]
In general, in these situations, it seems that the greater tyrant always seems to win, to the detriment of the community.
A society with free speech is supposed to tolerate the expression of really bad ideas partly because nothing other than the consent of human minds lets society distnguish good ideas from bad.
But no particular organisation has a duty to be a forum for all ideas. And that's why it's OK to kick a racist out of your dinner party, and OK for reddit to try and censor its worst users. But when such organisations are main forum for conversion, then the censorship starts to look total.
I think there are worse sources of missinformation, like Redit.