5 million concurrent users, connected at the same time, sending and receiving data through a persistent connection. Phoenix itself got 2 million on a single node: http://www.phoenixframework.org/blog/the-road-to-2-million-w...

They likely have much more than 5 million users.

To put this amount in perspective, you get 5 million connections after receiving 3000 connections per second, which are never dropped and remain connected, for ~28 minutes. The majority of websites do not get even 300 requests per second.