Someone you see at a bar or a club who looks really familiar but you can't quite remember how you know them or where from. As the night progresses you realize you may be friends with them on facebook or have just seen them in pictures on facebook because you are that creepy person who looks at everyone's photos and profiles.
"Dude that girl looks really familiar but I can't remember where I know her from."
"Are you sure?"
"Yeah, shes probably just Facebook Familiar"