They can block sites. But as I mentioned, I was told facebook isn't one of them. Maybe it is in some libraries. I know in my library at home, everyone uses all the computers. There are only 5 of them. Somr stuff doesn't come up on all of them, but they are all the same and next to each other. I've been in a lot of libraries, and the ones I visit are pretty much like my home library.
I guess I am more spoiled with my library then I realized. I checked the 6-12 and 13-17 computers earlier today; I could get onto my FB from the teen pcs only.