Raptr is one of the gaming social networks that seems to have some staying power -- the idea is that you can broadcast and track what and when you're playing, and the network will share all of that information with your friends as it happens. Raptr has just introduced an iPhone app that allows you to check in on all your friends and their gaming habits from anywhere, and even send messages across to Xbox Live with just one inbox.
The app is free on the App Store, but if you're brand new to Raptr, you'll want to head over to their site and create an account first (and probably pick up the desktop client, too, although it's Windows-only at the moment). If you're already a Raptr user, then there's no reason not to have this app.
That said, a few recent App Store reviews say the app has a problem with friends list loading. I didn't see any issues, but be warned. I'm sure the Raptr team is hard at work on fixing any issues that come up.