The Swapnote software has been the go-to feature for friends to exchange notes, drawings, and photos on the 3DS. As of last night, though, Nintendo has disabled the SpotPass functionality, which allowed registered friends to trade notes over the Internet.
It decided the measure was necessary after an investigation into misuse of the feature to distribute offensive material, particularly when minors were involved. In a message last night, the company explained the decision and apologized to users who have enjoyed it properly.
Nintendo has learned that some consumers, including minors, have been exchanging their friend codes on Internet bulletin boards and then using Swapnote to exchange offensive material. Nintendo has been investigating ways of preventing this and determined it is best to stop the SpotPass feature of Swapnote because it allows direct exchange of photos and was actively misused. ... We are very sorry for any inconvenience to the many consumers who have been using this service responsibly; however this decision was made considering the point that many minors also use this feature of Swapnote. Thank you for your understanding.
You can still exchange notes via StreetPass, which requires you to be in close proximity with the other party, but the SpotPass feature was much less limited. Nintendo itself has used Swapnote on occasion, to send out promotional messages regarding upcoming games. Those will presumably cease as well.