I previously wrote an article about last.fm, my current favourite networking site. The concepts are very similar. Raptr detects what games you play by downloading their client and updating your details, it then updates your profile status (e.g. what game you're playing and what achievements you have made in that game). It allows you to add games to your library so people can see what games & consoles you own. Based on your own library and games you like, it'll recommend games to you that might be of interest. You can add fellow gamers and keep up-to-date with their gaming activity and compare your gaming tastes.
I've already recommended it to my oldest brother, an avid PC gamer that uses Steam, and he was very intrigued! It's still a Beta version and has a feedback console on the side of the page, allowing users to give thoughts for room for improvement. I can definitely see this gaining popularity as the gaming industry is vastly growing and gaining more share in the entertainment world.
If you're interested you can find my profile here!