I used to play actively here http://www.civplayers.com/
we have leagues set up for civ 3,4 and 5. Originally we used gamespy arcade's pre set league system, but it just wasn't set up properly so we created our own league on www.myleague.com(/civ3players) which was a site that host leagues for darts and billiards. Worked fine for a while until they decided to start charging us a few years back. At our height we had around 500 active players. Until there was a split in the membership. One of the site admins decided to make his own site and run things the way he wanted, taking almost half our members with him. Didn't last too long, but it made a lot of the older adult players stop playing. They didn't care much for the drama and epeen measuring.
Then a few months later civ 4 was released taking a good 60% of our membership initially. About 10-15 people that went on to civ 4 came back saying it was junk. Me included xD
Civ 3 is the ultimate online strategy game. We played ffa's a little differently. League rules for games we called Cton's were that you can't contact or form alliances with other players. Strictly everyone against everyone. Most for ancient era start games, and they were almost always played on one city elimination setting. Made for really intense games. But we also had a huge group of players that liked to played future start scenarios with one king regicide mode on. With cruise missles and nuking it was insane if you had people that actually knew what they were doing. The community is almost completely dead now though. This time last year it still had at least 40 or so active players. Now every time I log into gamespy the lobby is just plain empty. As it should be though. Old ass game, lived a lot longer than most ^^