User blog comment:Lizaj3/Some Questions/@comment-4995735-20121205170109

Many players confuse the Friend Code and Inviting Friends. Understandable, since the devs never really explain the purpose of the Friend Code. You kind of have to figure it out on your own. So I will attempt to offer a longer explanation for the newer players to clarify these two features.

The one-time use Friend Code is suppose to be for the person who invited you to join the game. It gives a reward for the referral. The code you enter is under the Sociality Button, Services tab, Invite friends (click the big arrow to the right). The code at the top is your code for them to enter on their island. To enter a friend's code, go to the Services tab and choose the Enter friend code arrow. As you discovered this can only be done once.

Inviting friends is basically just adding friends to your list so that you can visit their island. All you need is their unique friend name. Click the "Invite a Friend" button at the bottom of the Friend List and the request will go to them to accept or decline. Note, if they are not an active player the request will time out (I think the limit is 2 days). Once they are on your friends list you can go to clean their island.

~ radmand2