Interestingly MUDs still have some niche to fill. Granted, a lot of them withered roughly a decade ago when MMORPGs became mainstream, but there are some that are fully alive and kicking. For example, Aardwolf MUD [1] seems to have about 300 players online most of the time, with peaks of 400 players [2].

[1] telnet: aardmud.org 23 [2] http://www.aardwolf.com/stats.html