I think Facebook could be very intrusive to ones privacy as well as many other social networks because no one knows who can be looking at your information. It could be some stalker or some serial killer or who knows and so we should all be careful not to let too much information out about ourselves on Facebook.
Of course, Facebook infringes upon privacy. In fact, before Mark Zuckerberg became famous, he vowed to make it a point that no one had complete privacy. Zuckerberg wrote this message in one of his college notes before he became famous. The platform of face book mandates that privacy issues exist.