Mudding has been a large part of my life for several years now. I'm not going to explain what a mud is; there's a very good faq on the subject and it'd be silly to duplicate the effort.
I mud for two reasons: roleplaying and socialising. They're not easily separated, however; most of the socialising is talking about roleplay, and the point of roleplaying instead of doing something like reading is that you're doing it with others. Plus ... when you get right down to it, I have plenty of chances to be social without logging into a mud, and in the last year I've also gotten a lot of chances to roleplay face-to-face. So why this dogged determination to keep mudding, even when I don't really have time for it?
I'm not sure. I think what it comes down to is 'mud interaction is _different_'. Not necessarily better, but different enough that it allows me to explore a completely new set of possibilities. I've never managed a face-to-face character with the depth and richness of my mud characters, although over time a few are starting to come close. I'm beginning to think that the depth issue is just a function of the time compression on muds ... but I won't go into that here, or at least not yet.
I started mudding in the fall of 1991, with PernMUSH, although I had previously played various AberMUDs and logged into TimeTraveller MUCK once or twice. I played Pern consistently for several years, mostly as Deirdre and Alec, and learned a frightening amount about how to manage people and organisations. Frightening, because I never imagined that being a Weyrleader or a CraftSecond would translate into the real world. It does, though, and more smoothly than you might think.
I played other muds at the same time as Pern, such as the fondly-remembered Belgariad, and the less-fondly remembered Moo-Cow, er, Mua'Kaar. None of these really held my interest for long, however, until I discovered AmberMUSH in November of 1993. I found Amber fascinating; there was much more room for conflict in the genre than on Pern, and thus the opportunity for me to explore a wider variety of character concepts. It was on Amber that I learned how to describe my characters fully in as few sentences as possible; this taught me a great deal about word choice in poetry and prose.
This focus on Amber lasted until sometime during the spring of 1995. At this point I had moved halfway across the country and gotten a real job, so I had little ability to concentrate intensely on roleplaying. I played Amber sporadically during that spring and summer, but my heart wasn't in it. For a while I thought I might quit mudding entirely -- but then I found Garou. Found again would be more accurate, really, as I played there right around the same time I found Amber. I'm not really sure why Amber won out over Garou; theme reasons, partially, but it was also due to the fact that many of my old Pern friends were migrating to Amber at the same time I was.
Regardless, I started to play Garou fairly regularly, and grew quite attached to my characters there. This lasted for about a year, at which point the call of Real Life again became too strong and I drifted away from mudding again.
At the moment I only manage to play Pern; my primary character (Katany) is now Senior Weyrwoman of Igen, which keeps me busy enough doing admin work that I rarely get a chance to RP. I can't really complain though; I signed up for this, after all, and it's enjoyable in its fashion.
There's really only one social mud I log into regularly, and that's TooMUSH. TooMUSH began as a way to get the Narnia wizards (of which I was one) to work on Narnia instead of using wizwall to talk about PernMUSH. It sort of worked; we neither talked about PernMUSH _nor_ worked on Narnia.
Eventually Too became a place that people hung out when their mud-of-choice went down. And, for some of us, it became a place to 'backchannel' about our roleplaying muds -- to talk about what our characters were doing, organise events, share information and do more than a bit of gossiping about the players involved.
I spent about two years using Too as a place to backchannel, first for Pern and then for Amber. Now, though, Too is just a place I log into in order to talk to my friends about how my day is going. It's gone from purely oriented on roleplaying (sure, it was social, but it existed for us to talk about roleplaying) to almost purely social. I know a lot of people still use it to backchannel, so it's nice to think it's serving different functions depending on desire.