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1.
when two people, dating or not, proceed to comment on each others walls and pictures,back and forth continuously to the point of exhaustion.
"Maria commented on so many of Andy's pictures last night. She's totally facebook humping him."

"I feel bad for Maria's Ex"
"Why?"
"Because her and Andy are facebook humping in front of everyone"
by Hchamp May 30, 2007
 
2.
When someone comments or likes any status on a specific persons facebook page. It is also common for the individual to tag them in any possible status of their own to get them to notice.
For the past few weeks Wanda has been Facebook humping Sand's page. She might as well just call him and get it over with.
by WBMike June 26, 2012
 
3.
When you write messages, comment, and otherwise stalk a Facebook friend you have a major erection for, but you are not a close friend outside of Facebook.
Nancy wants to be friends with Connie in real life, but Connie doesn't want to. So, Nancy stalks Connie on Facebook, and otherwise attempts other methods of Facebook Humping.
by AnotherDudeOverThere September 11, 2011