If you want to build a social app, especially one that works tightly with facebook, you're going to end up building basically your own social network that synchronizes with facebook at various points in time (for comments and whatnot) and also performs feed fanout/distribution. If you anticipate hitting scale, this can be tricky to engineer properly. Facebook clearly has the framework developed for doing all of these things but since their API lacks a real strategic purpose (other than to trick companies into a hostile dependency with Facebook), it is under-designed and frustrating to use.
If Facebook were to offer a service where they would charge some amount of money for an app to get an app-specific feed of friends, feed of yourself, list of friends using an app, hosting/indexing of open graph objects (read: just the metadata), and notifications on any activity related to your app, I would bet companies would pay. You can do all of this today, but it's a hodgepodge of FQL, periodical scraping, your own distribution, and gross overreaches in privacy.
In other words, if they actually ran the Graph API like a business and not some poorly disguised attempt at being "Open", Facebook could a) add another revenue stream, b) increase usage/engagement on their own platform, and c) save a lot of developers' time.