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1.
adj. describes someone who is not quick-witted enough to come up with funny comments in real time in response to what people have to say, but is very capable of coming up with funny things to say to people online.
People who are facebook funny must be given the spatial separation necessary to diminish the effect of a temporally delayed comeback ruining the comic relevancy of their jokes.

Todd is a real drag to be around when you talk to him face-to-face, but if you post something on his wall you can expect him to come up with something pretty facebook funny within 24 hours or so.
by cincyhobofan2 July 29, 2011
 
2.
When someone appears to have comedic talent, via their Facebook Status Updates.
These people usually get many likes.
This term applies if you don't really know if the person is funny in person, or just on facebook.
"Do you know who Jacob R. Clark is?"

"Well.. I've never hung out with him, but I have him as added him on FB. All I know is that he's Facebook Funny."
by GertrudeT333 December 04, 2011
 
3.
Something that isn't really funny in the real world but it becomes funny in the context of facebook. The only reason it is considered funny is because it is on facebook. In the real world said something is probably lame.
Person 1: Didn't you think that was funny?!?!

Person 2: Funny? No. Facebook funny? sure

facebook
by WARonSOCIALnetworkingSITES October 19, 2009