Facebook is my most visited site but I'm not willing to rate it fully until they stop allowing dangerous sites to advertise. It is inexcusable. I can't imagine how my personal and other data would have been exploited had WOT not warned me about them (2 so far).
Facebook can be safe if used properly but unfortunately most users aren't computer experts thus can get themselves in to trouble. Facebook has been selling more and more of their ad space to phishing and scam sites betraying users' trust. Don't install applications from Facebook unless you want to pay the price.