Here you can discuss the clubs with the largest fanbases.

http://www.leaguefootball.weebly.com

has a lot of attendance figures around the world. I didn't expect Iranian clubs to be so huge, and the Brazilian league seems to be a little disappointing. Ofcourse the English leagues are the best supported leagues in the world, not a big surprise. LFC attendance is great, but with the new stadium coming (2050?) it might rise a lot.