A combination of the words Facebook and slut. It pertains to a person who over uses Facebook or a similar social networking site. Some one who posts too many pictures or comments on other's (or worse their own) pictures. Someone who updates their status 25 times a day or someone who spams others with useless invites.
"I am so sick of getting Sally's invites to these fucking stupid public action groups, why is she such a faceslut"