Spotpass shut down with Nintendo Network, however I don't know how far archival projects have gotten. StreetPass on the other hand still works, but you need to know someone else with a 3DS. StreetPass is only used for explorers so you cannot visit other people's islands.

You can also use Pretendos StreetPass relay and NetPass

You can't send your own miis that you have created yourself. You will need to send a traveler to that island using Streetpass. (This includes the netpass homebrew app.)

how do i get the netpass app tho