Well for facebook it seems quite clear, because they want to get the brands to use more facebook as the best way to communicate with their user (and thus spend their marketing budget for facebook campaigns). Most of the other sites in this field are more keen about the unique content which means in return more Google traffic (longtail).
So these sites are doing the job most forums where doing in the past and it seems to work (so many people must have so much time…). Anyway I really think they are useful in terms of answering everyday life questions. But for the questions that need expert opinions and thus quality answers this just doesn’t work. If you have ever looked at the answers in the medical field or any technical sophisticated field it is quite scary (sometimes even dangerous) what you read.
Expert sites are kind of the answer to this problem, but they try to charge for an answer I don’t know how useful it might be (upfront payment). I think the companies itself should support places for answering very specific questions for free and let the brands pay for the service. This would be better for the user, the brands who can position themselves in a very specific situation and for Google traffic anyway. I am 100% sure that people are more likely to refer to an expert opinion than to another Q&A site and this means that these links could drive even more traffic to the expert site.
Focused expert Q&A site
A focused site doesn’t require much work in the long run (expect for the medical field). You just need to post the answers public and after a while I am pretty sure most of the questions have been answered and you just need to answer with an email that has already been written before.
Here is an intersting post about Quora, a larger site from the US.