A quick follow-up to Joel's answer:
Joel, Alex and myself met with Don Faul and Charles Dowd of Facebook last week, to discuss ways in which we could work together to improve developer relations.
We expressed the concerns laid out here: that questions aren't getting answered by Facebook, and that the (lack of) documentation makes it difficult for anyone else to answer them.
They've pledged to make a real effort to getting more questions answered on Stack Overflow, both by increasing the answers posted by the developer relations team at Facebook, and by working to keep the documentation more up-to-date. We may also do some sort of SO dev <-> Facebook dev meet-up in the future, to help familiarize their engineering team with how participating on Stack Overflow can help developers gather feedback on their work.
All of us agreed that a big part of the problem is the large number of low-quality or straight-up off-topic questions posted to facebook. Some of this is unavoidable - as traffic increases, so does noise; fortunately, Stack Overflow provides everyone with the tools to help clean this up. Some of this has been due to the ease of use of SO in contrast with Facebook's own support forums (combined the occasional lack of clarity on Facebook's end as to what actually belongs here). They're going to work on providing better support options for questions that don't involve programming.
My gut feeling is that Facebook is in a situation today somewhat similar to Microsoft in the early to mid '90s: a popular, fast-growing platform, with documentation and support for developers lagging behind. It took a while, but Microsoft caught up - by making developers a priority, even as their income came from sales of Windows and Office. Facebook needs to do the same.
Finally, thanks for holding our feet to the fire on this. Those of you who are active in the [facebook] tags are the heart of this community on Stack Overflow - if you're feeling slighted, overwhelmed or powerless to effectively moderate your own community, fixing that is a huge priority for us.