Your experience on FaceBook sounds more like MySpace. I've never had a friend request on FaceBook that turned out to be a spammer peddling porn. I have had a few friend requests from authors who write in genres I like so I haven't minded those.
MySpace is another matter. Until I tweaked my profile to require a full name and an email address, I would receive 3 - 5 friend requests daily that wanted to lure me to porn sites.
I'm not a cheerleader for FaceBook and won't try to convert you but I can see the potential in an academic setting. That's where the students are. There are also interesting apps being written for FaceBook including some that provide connectivity to library resources. Why a library app? If the students are going to be there anyway why not give them easy access in the event they have a sudden urge to look something up.