The act of taking a picture that contains a funny or strange face and photoshopping that face onto the other people in the picture.
Guy: Hey, check out this picture of Kyle at the party. He looks like he shat himself. I'm gonna facebomb this.
-some time later-
Guy: hey, look what I did.
Girl: Oh my god, they all have that face, thats so funny!
Guy: Yeah, these party pics are facebombing gold.