Well I've thought about it numerous times because I rarely use my Facebook these days.
The main reason why I haven't deleted it is because a good nber of my old friends occasionally connect with me through Facebook. Just the other day my classmate from waaaaay back in highschool sent me a message and we got to catch up.
As much as I'd like to delete it, it's still a pretty good avenue for people to connect with me, so that's basically why I still keep it around.