When you think you know someone, but then you realise you've just seen them on Facebook... sometimes via a friend or sometimes via Facebook Stalking.
Guy 1 "I'm sure I know that girl..."
Guy 2 "I think she's just Facebook Familiar"
Guy 1 "I said 'Hey' to this girl the other day, I was sure I knew her from somewhere... but she disagreed"
Guy 2 "Facebook Familiarity - It's a bitch"