See also the blog.stackoverflow.com side of the partnership here: http://blog.stackoverflow.com/2011/08/facebook-stackoverflow... Basically, it's a siloed version of stackoverflow.com containing only the tags relevant to Facebook. They're thinking about creating mini-sites for other major subdomains (see also http://meta.webapps.stackexchange.com/questions/913/is-this-... ).
Hate the idea. SO's search is decent. This is just the kind of fragmentation that I already don't like about programmers. , unix. etc.. You can easily filter your lists by tags, you can easily search by tag. Where's the benefit?
I'm not sure that this level of filtering is quite necessary, but I think that the community fragmentation is useful: People go to each site (SO, SE, SU, Programmers, etc) with a specific purpose, and the topics on each site are broken up by tags. Trying to filter both purpose and topic on one site would be problematic.
(Also, the Unix/AskUbuntu/SuperUser debacle is a special case, IMO)