Also, the system is aware of which friends are online and what they're playing.
http://www.nintendoworldreport.com/news/24772
At the Nintendo Europe 3DS event in Amsterdam, details of the system's online connectivity were finally revealed. Based on a brief comment on stage at the event, we now know that there will be a single, unified Friend Code per 3DS system. Once players trade their codes, they will be able to connect in all online-enabled games -- there are no game-specific codes as on Wii and DS.
Also, there is a system-level feature allowing you to monitor which of your friends are online with their 3DS systems, and which games they are playing. This will help you to coordinate online matches and build a sense of community. We don't yet know if this feature will also be available within games, or if it is only available on the system's home menu.
UPDATE: Users will also be able to swap Friend Codes with the push of a button if they're connected via local wireless.