Language/platform choice explains why they need 50 engineers rather than 100. The trivial scope of the application explains why it's 50-100 and not 5000. It's a messaging service! If they weren't extending their application by developing new functionality then 50 even sounds like a lot (assuming they don't run any of their own hardware).
Don't forget they also have to develop the front-end applications, which include iOS, Android, web, ... That will probably be the largest part of their engineering effort.
