When it first started out, it offered a clean, no-frills interface. It wasn't about blasting music to visitors or picking color schemes that included obnoxious animated gifs.
The other thing is when it first started out, it was exclusive to colleges. Everyone wants to be part of the in-crowd, so an exclusive website had a certain kind of allure.
Now? Everybody else and their mother is on facebook, so people who aren't using it yet typically join up because their friends and family are using it.