I've put over 40 hours into Street Fighter IV on 3DS, but I wonder how many people are still playing that game online.
I have 100+ hours over two years. Online is pretty populated as well.
It really helps that SF4
still has the best implementation of online of all fighters on 3ds, in that that you can start singleplayer campaign and set it to accept multiplayer requests while you're playing. On other fighting 3ds games i own (Tekken, DOA) you go to lobby and just wait staring on a screen.
Even these days, i won't be able to finish one round without at least one challenger to appear, even if you set restrictions such as control method, region and skill.
Such a brilliant game all around. The only problem is that they should have set timers restricting constant spamming of projectiles on touch mode, which is easily solved by setting control mode but still.