SupCom's still active, through Forged Alliance Forever. I got back into it a couple of months back and was pleasantly suprised by how easy it was to find a game.
I lost interest in FA around the time when FAF was in its first betas.
Planetary Annihilation is more or less a successor but I could not get into it.
I like its idea but in practice the micro was insane in order to keep an eye on several different planets.