In the city I'm in about 25% of the population has facebook accounts. That said my luddite survival metre is set to agitated by the very concept.
Apparently, even private correspondence posted to the site becomes the property of facebook and it is being used as a powerful market research tool. The U.S. intelligence services are also theoretically involved (shareholders), this last one isn't a bad thing of course - the more info they have on me the less likely they are to get confused and waste time (eg.
http://foia.fbi.gov/foiaindex/einstein.htm - You and I may not be famous but I'm sure that twenty minutes wasted from a paranoid could be better spent hunting Jihadist funding channels).