Why do all that experimentation yourself? Shouldn't it be done by web scientists once and for all? You could then use the results of their research as guidelines to determine the sort of app to build as well as its features/UI.Also see:
http://www.cs.umd.edu/~ben/ShneidermanCACM6-2007.pdf
http://weblog.fortnow.com/2007/08/impact-of-facebook-platfor...
http://www.facebook.com/topic.php?uid=2205007948&topic=1...