There are 2 different mods for Grand Theft Auto San Andreas online, Multi Theft Auto (MTA) and San Andreas Multi Player (SA-MP)
Last time I played San Andreas, I used both mods and MTA had barley any servers, SA-MP seemed to have hundreds if not thousands of servers, With alot of people playing. If you're using MTA, That could explain why you have no one to play with.
I have San Andreas Multiplayer and if you don't have a fast internet connection, it's pretty much unplayable.
PS: If you wanted to play it online, without having the game in your computer, you can't, it's too big to be an online game.