Another one of my pet peeves would be people who post unrelated comments on my Facebook status update or photos.
I don't expect people to reply, but if I say something on my status update, that means that's the topic that I'm currently interested in. If people do reply, then I expect them to stick with the topic.
For example, when I write "Anybody got some good vacation spot suggestions in Tokyo?" then I wouldn't want someone to reply, "Hey Iggy, have you bought that silverware set that we were talking about yesterday?" but somebody among my friends might do just that. If that's the case, I would delete his or her comment and post on his or her wall instead so the conversation can happen there instead. I don't want the purpose of my status update getting derailed.