My girlfriend uses the mobile web version of Facebook on a semi-dumb slider phone (it might be some variant of Symbian). I find it baffling because the browser is so terrible and the screen is 320x240, but she's perfectly happy with it. She never uses it for any other sites. I wonder if Facebook has an unusually high number of users like that. The demographic would be invisible to the rest of the web community because those users never leave Facebook.