Like a real Affair, a Facebook Affair is when someone is cheating on their boyfriend/husband or girlfriend/wife with someone on Facebook. The relationship never really becomes physical and the two people might never actually meet. The Facebook Affair ranges from them sexting eachother and having very sexual conversations like they are a couple, to flirting and giving eachother "cute" nicknames like "honey" and "boo".
This can go on for months or be a one night thing. The two people may actually know each other or be complete strangers, but either way when the person they are actually in a relationship finds out there will be hell to pay.
Jack: Hey, did you hear Bill got fired?
Mike: What? Why?
Jack: He was having a Facebook Affair with his boss's wife. She left her profile open and he saw their messages.
Mike: Damn, but she is a milf sooooo...
Jen: I feel terrible.
Kelly: What happened?
Jen: Mike and I had a fight lastnight and I got drunk and had a Facebook Affair with an Italian Underwear Model.
Kelly: Damn, but Marks a douche sooooo...