Well there used to be SwapNote, but they've already removed that. I hope that Nintendo implements a new way to communicate with our friends, but I don't know if they have any plans to do so. Probably not.
I really liked SwapNote. The 3DS was the only way I could talk to some of my older friends who don't log on to social media sites anymore. Since they're gaming, they could just see the messages via SwapNote. I lost contact with many people after they removed it, but they did so for good reason, so I guess I can't complain.