Imagine how long it would take to send a simple message to all of those websockets. Having many websockets is nice, but only if the rest of your infrastructure can deal with it.
On a perfect network, I presume. In reality, clients lose packets and strains the system differently. There is more to this than just perfect network conditions in a lab setting.
Clearly a chatroom with half a million individuals is unusable from pretty much every perspective. That said, a chat server with N chatrooms and a total population of 500k users sounds like a good day on IRC and well within the realm of what something like this could potentially handle.
That depends very much on what's doing the chatting. If it's code chatting with other code -- for example, mobile devices receiving near-real-time notifications -- then half a million is just getting started.
