For the record, I'm talking about PSN.
I do play Free Mode on TBoGT from time to time and it's usually pretty crowded; probably around an average of ten people each time I join. GTA Races and Races usually have a good amount of people for you to pick up a decent race. Deathmatch and Team Deathmatch, for me anyway, are always dead. If I'm lucky, I'll get a game of 2 v. 2, but with non-serious players.