Half-life and valve itself has a great following because they've setup their multiplayer so well. People can create custom maps, mod their servs, kick and ban people, do all sorts of things, so the game never really gets old. Plus, there are more and more new people that just got into the game, keeping the game alive, and replacing the old ones that got tired of the game and left That's why it lasts so long.
And, for one, Valve uses it's own online service (now STEAM), and not GAMESPY. I've never been a fan of gamespy. Lots of advertising, blah! It's good though that you don't need gamespy to get into the starwars games. And with the recent patch, they improved the game finding interface with filters, etc. They're getting the hang of it.
Also, games last a really long time if the company that made the game keeps supporting it with new versions. Mechwarrior 3 was a great game, released in 1998 you'd have tons of people on MSN Zone, in the hundreds, all to play the game. However, the studio that developed the game only came out with 1 patch, 1 measly patch that helped a little with the gameplay. There were custom maps, and some other customizations, but very few and far between. Their multiplayer thing could've been better. They could've had the option of dedicated servers. They didn't have the option to join while the game was in progress, which really killed it. No patches, no custom stuff, the game pretty much died after a while. And then windows XP comes out, and the game doesn't even run on the new OS anymore. There are still a few die-hard people who play mw3, the room hits around 20 people or so, with half of them away and a quarter of them playing. LA needs to stay on top of this game, and keep it going, and it'll have a long future
Oh yesh, did i mention i love dunkin donuts?