Someone with whom you converse on a semi-regular basis via Facebook, but almost never in person. Should you legitimately meet said acquaintance face-to-face, neither one of you will act as if you know or have ever talked to the other.
-Hey Gil, are you ever gonna get the cahones to ask Katie out?
-Nah, I don't know how to approach her and talk to her for real. We're only Facebook acquaintances.