The only multiplayer game I would play minus my friends is FIFA online.
Call of Duty: I have a group of friends I met all the way in CoD4. If they stopped playing today, it is almost certain that I would never get another Call of Duty.
Borderlands 2: My step-brother/friend who moved away to California is whom I play. If he stopped playing, there is a good chance that I would stop playing (I do like the games so I might finish it).
Halo Wars: I am one of the five people still playing this game. However, I only play 2v2 or 3v3 with my two friends who also own this game. Once again, if they quit, it's certain that I would stop playing.
Madden 2013: I play a franchise with the Halo Wars group. If they quit playing, it would be likely that I would stop playing Madden online, though I would continue playing singleplayer.