Etherpad uses socket.io.

Here's one hosted Etherpad: https://etherpad.wikimedia.org/

Dasher (http://dasher.im), uses socket.io. We're using socker.IO-objc for our iOS client: https://github.com/pkyeck/socket.IO-objc

And will be launching a web client in the coming weeks.

So far, so good. We do handle the reconnects ourselves, but otherwise it's been treating us well.