Facebook is a place where most of the people you encounter have a real-world persona that pretty much matches their online persona, so if you take care to know the people you friend, it cuts out a lot of bad behavior. You have the opportunity, though you can also screw it up with poor choices of friends, to have a community that avoids much of the trouble of random corners of the internet. People are *much* better behaved when their real-world friends and acquaintances can see them.
This is useful. It doesn't serve every purpose, and it isn't intended to. It isn't for everyone, and that's okay. It discriminates against people who would otherwise behave behind a pseudonym, so it's not perfect. Nothing is.
A place without pseudonyms is useful, even if it isn't universal.
Disclaimer: My slashdot name is a pseudonym. My facebook name isn't.