Of course, you're not paying them for their effort. You're paying them for creating a distribution channel you can access, and then the ongoing product development that maintains and grows that distribution base.
Linking it to subsequent effort is like saying a programmer should only charge $1 for all bar the first copy of their program, given how easy it is to burn new dvds.
Actually reading this I wonder why facebook gave applications the ability to collect email addresses easily? Lowers platform lock in as someone like Zynga if they do move away from facebook can just use the email to keep the user base they have acquired.
They had to give the FB app developers some alternative to newsfeed notifications to keep them from getting to enraged when their primary source of recruiting users was yanked out from under them.
Linking it to subsequent effort is like saying a programmer should only charge $1 for all bar the first copy of their program, given how easy it is to burn new dvds.