You either get Pokemon from other people or by using SpotPass. You're limited to three new Pokemon a day via SpotPass. Keep in mind that StreetPass should not be confused with SpotPass. SpotPass uses an active Wi-Fi connection to download the new Pokemon. StreetPass, which has no active functionality at this time, is for when you pass other 3DS systems in Sleep Mode.
Anyway. It's given renewed life to the system. Silly that I'm more interested in this "game" than anything else at the current time.
Related Link: Pokedex 3DS for the Nintendo 3DS