Speaking for my own use-case, it's a mix of founder's statements (http://gawker.com/5636765/facebook-ceo-admits-to-calling-use...), investments by less-than-pristine Russian businessmen, my own various intersections with data mining, personal information aggregators, and legal processes, history of wide-spread domestic spying (watch "The Lives of Others" about the East German Stasi for a chilling view, understanding that this has and does happen in many lands).

The conceivable downsides of open participation in Facebook far outweigh the positives.

Add in social fatigue in the mass market as well, I suspect the trend has peaked.