Is there a scalable open source web BB/forum?

Some places say phpBB doesn't scale past 5000 concurrent users. Is there FOSS forum software out there that might go further, to say 7k-10k or more?

From jldugger's original link:

phpBB falls short primarily on scalability, however until you reach about 5,000 active members, you should be okay.

From Gaia Online, a phpBB-based web forum, as of 25/5/09 (today):

28985 users. (24975 visible, 1664 hidden, 2346 guests).

29k > 5k! Now perhaps the original link was talking about scalability out of the box without tweaking, but it didn't say that and so I'd treat it with some suspicion.

Some more handy links:

• An admin of Gaia Online detailing the tweaks s/he made to deal with their high volume of users and posts.
• A list of very large boards along with the software runs them.
• Phorum: FOSS forum software with a reputation for performance.

I know it isn't open source but you may want to look at vBulletin it is what is used to run (http://ubuntuforums.org/) which is a pretty active forum. You do get the source, you are just not allowed to redistribute it.

I like the Simple Machines Forum. One could add memcache as internal cache supplier and use another instance for sessions, allowing the whole thing to be spread over multiple loadbalanced webservers.