I posted this a few months back on a different thread regarding Facebook (http://www.theyeshivaworld.com/coffeeroom/topic/facebook-and-shidduchim). I took the liberty of editing myself a bit.
IMHO, there is nothing wrong with Facebook. You can argue that people can end up doing stupid things and getting involved with people they shouldn’t be getting involved with, but you can say that about everything.
If someone wants to do the wrong thing, they will do it, regardless of whether they have Facebook or not. A responsible, smart person would not do stupid things. A responsible, smart person would not accept a friend request from a stranger or some guy. If they are stupid enough to do that, they deserve whatever happens to them and the reputation they will get. (Sorry,it really bothers me when people do stupid things so I tend to get worked up about it)
Just be smart and do the right thing and you will be fine. And if you can’t trust yourself then you have bigger problems than Facebook.
Just to add- I understand what Logician is saying about not putting yourself in a compromising situation. Walking out of your house can be a risky thing. Having a cell phone can be too. But you have to be able to set your own boundaries and know whether or not you are able to trust yourself in the possibly compromising situations Facebook might pose.