Excerpt from a 1 vs 100 developer post:
"Two years ago, our team was asked to develop and implement a new kind of game. Our guiding principles: it had to be scheduled like a TV show, it had to be social, and it had to support hundreds of thousands of players at once. "Above all," we were told, "it has to be big. It has to be something nobody has seen before. Go big or go home."
Now, as we've wrapped up 1 vs 100 Beta and are getting ready for launch, I think we've accomplished that goal. We've faced some big challenges along the way, and done some things no game has ever done before. We've hosted a game with over 100,000 concurrent users in the same session. Doing that required designing a new server architecture, different than that used by traditional MMOs--we didn't want to shard groups of users into their own sessions on their own servers. Everyone gets to play together!
Speaking of servers, 1 vs 100 is synchronous in a way no other game is. Hundreds of thousands of users playing a game show together means that most of the players, most of the time, are making the same server calls. 1 vs 100 is so synchronized, in fact, that everyone is making those same calls within a couple of hundred milliseconds. Our server was designed to handle that kind of traffic. Some of the other services we use are not; they handle heavy load by assuming that requests are evenly spread out over time. This required some fancy footwork on our part not to overload our partners' servers."
http://www.xbox.com/en-US/live/1vs100/blogs.htm