We've only needed to handle concurrent connections in the 1-10k range though.
There are really better c++ and erlang implementations we've investigated but won't pull those off the shelf till needed.
Obviously there's loads of great realtime-as-a-service offerings like Firebase and Pusher that make sense for a lot of use cases, just not ours.
We were always uncomfortable with relying on a third party for a core aspect of infrastructure, and this acquisition introduces uncertainty that we don't have to worry about.
 Poxa: https://github.com/edgurgel/poxa