I can say that for a good portion of my gaming life I've been circling 3 games - Ragnarok, WoW and Counterstrike Source.
A few times I've gone back to Ragnarok Online because my mate also has, but then it's like he's not on a whole lot and the boredom sets in. Initially I was playing it for ages back on a private server where I knew everyone.. but that's long gone.
I haven't returned to WoW that much, but when I have returned.. it's been for long periods. It wasn't really for friends either, though I did make friends on the game. I returned a couple of months after the burning crusade (obviously the wrong time to return.. i should've returned asap so that I didn't completely fall behind...), but now I'm pretty sure I won't be returning again, I haven't played it for over 2 years.
I've always gone back to Counter-strike source because of friends, in fact I've been playing it on and off for coming up to 6 years (it came out November '04, and I had a steam account in '04 so chances are I got it around that time). It's great because I've played it in small amounts but overall a huge amount of time, and still for the one-off payment.
I prefer getting new games to play with mates rather than going back to the old 3. Like when I went to uni I played Halo 3 with over 10 mates in the halls of residence (which was great), and atm I'm playing Starcraft 2 with at least 4 mates.
