Mostly unnecessary wall posts, usually just for attention. The worst facebook foul's even wall post to people they have no real connection to.
Another component of a facebook foul is liking or commenting on things that the said person has nothing to do with. Wall posts for conversations are also a tell-tale sign of a chronic facebook foul.
John: "That's such a facebook foul"
N: That was a facebook foul. She's creeper status.