I played RuneScape for seven years. I made several friends whom I still keep contact with (we all keep in contact with each other, honestly). In fact, I'm probably going to a Nightwish concert with one of 'em in September. It's surprising what life-long friendships can be made on MMOs. That being said, it's easy to get addicted, or at least start playing to much. I used to play for at least eight hours a day, even on school days. It was quite fun, I'll still say that today, but I will say that it was unhealthy. But at the same time, some of the best friendships I made on there were with people that were the same way. You'd be surprised why some people play MMOs religiously. I think one common thread is depression. It's a way of escaping life when people are unhappy with theirs. You get a feeling of achievement from completing quests, getting that next high-level/expensive item, and you make friends in a much more ideal environment. So a lot of people who get addicted, in my opinion, need help in real life. Once reality is better, they'll migrate away from it.
The reason why I quit personally is because... well, it just got boring... I mean, it's RuneScape. I've tried other MMOs like WoW, RoM, and about five others, and while they're fun, it's not the same when you don't have a clan or guild. All of my RuneScape friends, as I call them, are busy with their own real life things, myself included, and don't really want to get into MMOs at the moment. I probably never will again, but I'll always remain in contact with 'em.