Well going online on your own .. small chance that u'll end up in a populated server. It helps adding people that you do meet to your friends list, or just add a real friend that also has gta 4.. alot of times I got on full servers by just inviting some friends and more joined
U do have more change on clean servers on finding people, but you don't really need multiplayer to enjoy gta you know. Just put in some of your own music, install some cool mods and the game can be alot of fun in single player.
Lots of cool mods out there that can make your game so much more enjoyable