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1.
A man or woman that looks extremely good in their photos on facebook, but in person is extremely ugly. Easily fooled by teenagers. Original pictures often changed by editing or "piknicking" the photo.
Sawyer: Dude, this chick looks so hot in her facebook pictures.

Peter: Don't even bother man, I thought the same thing until I met her in person.

Vaughn: Guys, she's a Facebook grenade.
by Let it rain Mcilvain January 07, 2011
 
2.
A person that is not your significant other that you are friends with on Facebook that is always liking/commenting on everything that you write regardless of if it is something that they find amusing or interesting. Originated by Steven Garcia
Man I got this Facebook Grenade that has been liking and commenting on everything that I put on Facebook!
by lilg210 October 27, 2011