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The app is available in the app store [iTunes link] for free right now. While you can't use the app to play games (it's not that kind of app), you can use it to see what your friends have been up to across all the above-mentioned gaming networks. You can also send and receive messages through Xbox Live and Raptr's network.
Raptr — one of our top five social networks for gamers — serves up activity streams to let you know when your friends are playing certain games, or when they unlock Xbox Live achievements and other milestones. With the app, you can log in to see if your friend has beaten you to that Zombie Genocide achievement in Dead Rising 2. If so, you'd better rush home and catch up!