By far, the Messenger Plus! forums are the biggest forums out there that use MyBB (to my knowledge).
I've got 20,878 members but quite low on threads etc....more of a closed community so a lot of the functionality is outside the forums....and it's not listed on MyBBFans...
Consider yourselves put on notice.... there's a new Sheriff in these here parts.
http://www.ncaabbs.com/forums/ncaa/mybb
that's a demo, and we're still reorganizing and making it pretty, but you should be able to see the flagrant disregard for sanity in the size of the board, and its statistics