When you're out with someone, you should really make the effort to be with them . seriously. Just being there physically but at the same time you're checking out facebook or twitter or your mail is just plain rude. You're basically ignoring the person you're with and refusing to make conversation with them. I understand if there's something important you have to attend to but if its only to check on updates come on.... what's the point of hanging out with others if you won't even bother starting a converstaion with them.
This is particularly important when you're spending time with the kids.