More people—maybe twice as many—seem to be playing the original version of Counter-Strike than Global Offensive. Look here . Why hasn't CS:GO inherited its elders' popularity? I'd blame the map drought GO has experienced. New official maps have been hard to come by since the game launched in August, and the fresh ones introduced by GO were restricted to Demolition and Arms Race modes.
Adding CS:GO to Steam Workshop is one step toward a healthier multiplayer scene. Debuting today, the Maps Workshop has 157 entries at the moment, including remakes of stuff like de_piranesi, fy_iceworld, and fy_pool_day, which I have vague-but-fond memories of playing in LAN cafes.
Pop over to the CS:GO Map Workshop to start downloading directly through Steam. After that, you'll have to find a server hosting matches on the levels you've downloaded. Alternatively, you can take a map for a spin with bots by typing "map [mapname]" in console.