I've been running a flash/AIR MMO for about 6 years and make a living off of it. I'm using PlayerIO.com. The service is great, and they can handle everything from the server, to the database, authentication, file hosting, etc. However, the customer service is extremely lacking, and the service as a whole seems like a wasteland. For that reason, it's a little bit hard for me to recommend if your making something larger and you plan on it being a very serious piece of work. However if your looking to prototype or get some learning experience, then it's good for that. As of right now, I'm trying to learn AWS services to perhaps transfer, however it's quite a bit higher learning curve.