Most of the apps at the time optimized for pageviews and monetized through DoubleClick, AdSense and other third-party ad networks, so there was non-trivial task of proving that "making money" was happening, followed by further non-trivial task of actually collecting such revenue from all of the jurisdictions.

>> Otherwise, what's the point of building the platform?

Facebook displays its own ads on application canvas pages. Therefore higher engagement with apps => higher pageviews => higher earnings.