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Sexual encounters via facebook. Esp. if on a regular basis between the same people. This is done through private messages, facebook chat, or wall posts.
"I was up all night facebonking **** he sure knows how to please a woman!"

"Yeah I've been facebonking this girl for a while now, the other night we were chatting and my Mum walked in on us, it was so embarassing."
by Lyd8 May 03, 2011