One real problem with Facebook is that you have to stay active and aware of privacy policy changes, and keep informed on feature changes, or otherwise you may find that policy/features have changed and could result in divulging more information than you intended.
The same applies to many or most Google services including Gmail and G+.
Note that my information comes from reading the numerous news articles about Facebook and Google privacy. I have never been a member of FB, nor does it appear that I will ever join.
I find far too much of my personal information on the Internet, not due to any fault of my own but due to ever wider indexing of all kinds of information sources. For example, property ownership records. I was very annoyed to discover that anybody knowing my real name can now Google my street address and unlisted home phone number. (However the information is out of date, I sold that house and now I'm cellphone only.)
I'm reacting to what I feel is too much personal information on the Internet by not joining social websites, so as to not contribute even more personal data. I don't mind discussing personal information nearly as much when I can retain my anonymity, such as here in the DC forum where my member name does not lead directly to my real name.