Once upon a time (2010) I suggested to Provide a virtual URL hierarchy for tags, which was originally declined. Then almost one year later (2011) along came http://facebook.stackoverflow.com, which is really just http://stackoverflow.com showing only and requiring facebook-ish tags while hiding all other questions. So technically, this is implemented.
However, to my knowledge fb.so is so far the only such subpage. While we shouldn't overdo it, shouldn't we be able to request other such subpages as well where it makes sense? I bet we could help many people with e.g. game specific arQAde subpages, or other subpages for Area51 proposals that were subsets of other sites but would still benefit from providing their community with a specific subsite.