mybb capabilities

[ZiNgA BuRgA]

Depends what you mean.

Millions of registered users?
Millions of users online (defined by a ~15 minute WOL period)?
Millions of concurrent webserver connections?

Server setup?

What are you trying to aim at exactly?


In the first instance, it's probably more the DBMS responsibility to store data on millions of registered users.  For the latter, it would be more on whether your server setup can handle it.  For millions of concurrent webserver connections, you'd probably be looking at a Facebook-esque type traffic profile, where you'd need tens of thousands of computers to handle the load and various stuff I'm not going to bother going into.

Forum software just affects how much server resources it uses for handling requests.  It's not going to suddenly become erratic after a certain number of users.
For a benchmark of a few software, you can try this: http://bb-bench.com/benchmark/2