Because Facebook has so many users it's an extremely interesting place.
Personally I think it was even more interesting a year ago - because at that time third pary apps seemed to be virally working their way through the book with incredible speed... however, in the last year, partly because of Facebook API changes and partly because of Facebook users growing up (there are only so many people who want to play vampires vs pirates!) then the viral spread of apps has decreased.
One of the nice things about GoogleAppsEngine is that it makes it easier to write scaleable applications - hence easier to write applications that might satisfy the Facebook member's needs.
Looking around, a few facebook developers have spotted this - and they've posted some helpful material on:
And they've formed a group at: