They might also just not be the sort of people to comment on Facebook. I rarely comment on people's photos unless I'm actually in them, for instance. I don't have any family on there except a couple of cousins, though.
On the topic of people who comment on every single status update--I think I'd find that a little weird, though not necessarily annoying. My status updates usually get less than three comments, unless it turns into a discussion, and lots of them don't get comments (this is true of most of my friends' status updates too). So it would be very noticeable if one person commented on every single update I posted. I would probably just wonder if they were bored, though, and if they did this with everyone on their friends list!
Oh, I thought of another Facebook etiquette. Don't jump in to defend one of your friends on their status updates or wall if you don't know the full story. They might just be having an inside joke with a friend of theirs. Especially if you don't know the other person.