Once your character crosses to the mainland (or accepts the experience just preceding the cross to the mainland), you can no longer ferry people.
For most people, they can ferry others only once because they wish to carry on with the campaign. However, some people try to keep a character, for a while at least, that will never leave newbie island. Therefore they can ferry others more than once. This character will probably get played eventually, though, since it is impossible to carry on with the story if they remain on that quest.
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