I've run forums even on rickety DIY PHP + Postgres that's held up very nicely to several hundred logins and even more sessions per day on very modest hardware (1.5Ghz Core 2 duo, 2Gb memory).

It doesn't take a boatload of resources for a simple login and static content if it's just static content. Sites can be separated (or "sharded" if you like that term) on Nginx with very little effort for static content.

E.G. https://gist.github.com/perusio/2154289

There are more elegant solutions for that which still don't require blob objects.