I used to love getting Streetpasses and playing the Streetpass games, but after a certain point it was very rare to find anyone else carrying a 3DS, even at conventions. (And the rate was already much lower than in Japan where it was much more common.)
I had good memories with Swapnote, before they got rid of the ability to share notes. I made a whole month of Smash Bros. Brawl memes in 3D, before Smash 4 came out (made more annoying by the fact that I had to send my 3DS in for repairs in the middle of it).
I never really played any games online, but I got a lot of use out of my system. I actually used 3DS Sound as my music player for a long time, until I finally started using my phone.
Lots of great memories with various games and apps; Badge Arcade, 3DS Video (Dinosaur Office and BearShark were awesome), Ambassador games; 3DS was the first (and probably last) system that I bought at launch.
Official King of PotD
You only need one T-Rex to make the point, though. ~ Samus Sedai