Depends. When a new system launches, most people adds everyone. But as time goes on, people either run out of space to add all the new folks (like myself), forget to check this thread if they are still actively looking to populate their friends list, or are totally okay with who they have on their roster and is not interested in getting anymore.
My advice with the 3DS friends system is if you want specific people to mutually add you to their list, the best way to go about it is to PM them with a message that includes your FC and stating that you added them and want you on their list as well. It at least guarantees that you can get their attention.
Good luck