I think changing APIs is begging for a wiki much more than a Q&A site. Facebook used to have a wiki for their api, but they went and deleted it, which I considered basically unforgiveable and proceeded to do my best to avoid having to touch the API after that.
In short, bringing back http://wiki.developers.facebook.com/ and focusing on keeping it current would be a much better start than a Q&A site, even a really good Q&A site. You'd still have the need-to-keep-with-changes problem, but at least you'd have somewhere to start from, and wouldn't keep getting referred to out of date information when googling answers.
Yes I know Stack Overflow has community editable wiki question styles, but then you just compound the problem and now need to update 100 different 'how do I [x]' questions every time the API changes.