[jacquesm]

If you're building a large scale chat service you are implicitly also building a message queue.

And as for the high load you haven't actually experienced high load until you put this into production with a million users.

To make that clearer: you can design a system for any number of users, the only relevant question is how it held up in practice and as long as you haven't had a million concurrent users you just don't know (and probably it won't).