Websockets is a great transport protocol for bi-direction realtime persistent connections. And it's very easy to write your own application protocol or just use WAMP from http://wamp.ws.
In the early days of the realtime web, long-polling was used mostly as an underlying hack in the emulation of streams. However, RealCrowd's API isn't a hack. When clients make requests to the server, they are attempting to synchronize a resource. One could argue that this kind of interface is semantically closer to what the client is trying to accomplish than one based around pure message passing.
I'm not just being pedantic about it either. Just compare server performance of RPC over Websocket to RPC over HTTP at scale.