Mainly that's because a lot of the right questions haven't been asked yet (I could probably provide a few of those instead of searching then quitting when I don't find what I need), or the information is out of date. It doesn't seem like there's a critical mass of users yet for it to be solid enough to replace the manual.
Also note I'm not saying I need a physical book in my hands. Just something indexed with a table of contents with most of the info I might go looking for.
I appreciate the responses here and on Twitter though, guys. Your support (excluding my qualms wrt documentation) really does seem top notch.
See their blog post: http://developers.facebook.com/blog/post/545/
Their documentation remains independent, the alternative is insane.
(disclaimed: SE Inc. employee)
For what it's worth: I agree. I hate the StackExchange, and I want to get rid of it. But our support team is extremely serious about fixing it, cleaning it up, keeping it up-to-date, and using it as a platform from which we can distribute printed, old-school manuals. The idea here would be that appropriately tagged answers are automatically collated into a print manual; keeping the manual up-to-date would necessarily also ensure that the StackExchange is up-to-date and vice-versa. Think a QA-focused wiki, more than anything else.
They're working on this very hard right now, so I'm hoping that I have something concrete to point to, soon. If it works out, I think it might be a great direction for Facebook to go, too, both providing the social pressures necessary to keep lightly trod StackExchanges up-to-date, and providing a way for Facebook to possibly have organized, accurate documentation for the first time in forever.