This install has more than a million users.
Yeap! This installation is probably the best example of a site with thousands of active users. You can even see how it works in a sisterly fashion at http;//profiles.wordpress.org. Same user tables but different site/blog so to speak.
However, any geek is quick to point out that “WordPress doesn’t scale”. I’ve always wondered if that was just an urban myth. There must be some truth to it?
Probably those same people who said that Ruby on Rails doesn’t scale (and no, please can we not start that? Forget I mentioned it). :p
You could always try the multi-db plugin from wpmu-dev, when the time comes.
WordPress scales just fine. Is it perfectly and sveltely built for all applications? No.
http://ma.tt/2006/03/wordpress-and-lyceum/ is a good read about this from a few years back. 
BuddyPress doesn’t really suffer any of these as advantages or disadvantages, since it currently sits on top of an MU installation; it just kinda “is.”
Andy,
What type of plugins/multiple db’s/caching do you have enable to handle all the users of this site?
Of course WordPress.com which runs MU has scaled quite well, but I know they do some special things on the backend to make that happen.
WordPress (which is what I’ve done up until now) scales VERY well. I’m amazed at how much traffic I can handle on WordPress for $6 a month.
Object caching is a good starting point, then you can look into things like PHP caching for the server (eAccelator)
And when you get too big for one server, get a second and put the db/mysql on one, everything else on the second box.
Then go from there.
eAccelerator made a HUGE difference on my low end dedicated server (Celeron w/512MB RAM)
Andy, John… Everyone: Thanks!
After reading all these replies, I start searching everything related to what u guys said. Then, I found this: https://mu.wordpress.org/forums/topic/2039?replies=4
Best regards,
Anderson
@duquebilly
One of the issues with using the straight built-in WordPress role management to manage users is that the usermeta is cached in a way that doesn’t scale well.
If this site has millions I imagine that it handles well, are there caching plugins that should be used to optimize BP for large amounts of users?
