I refuse to use Facebook, because they have pursued a strategy of sharing data; multiple statements by high-level FB execs suggest that privacy is not a corporate value. Although I've had to set up a sock puppet to be able to see certain pages. :-(

I see Google as being a company that values privacy, but inherently knows a good deal more than I am comfortable with.

I have accounts on LinkedIn, G+, and StackOverflow that are strictly "professional persona", for the purpose of networking only.