These certainly aren't numbers to be sneezed at. I went to talk to members of both mod communities to find out what keeps them playing.
Its two most popular online multiplayer mods currently have a million or more active players between them - one, Multi Theft Auto, had 616,000 players in July (up from just 33k in February 2010), while the other, SA-MP, oscillates between about 15,000 and 50,000 concurrent players. It may be two Grand Theft Auto generations and 11 years old, but GTA: San Andreas is still very much alive.