Ruby on rails has an important number of questions on stackoverflow.com... And I think Rubyonrails has a very active community on this site.
Why don't we have a subdomain for rubyonrails ? (rubyonrails.stackoverflow.com)
What's wrong with the way it's going now? Why do you need an extra site? SE only accepts new site proposals that don't significantly overlap the scope of an existing site, with very few exceptions (most notably Mathematica.SE).
If you're talking about something like http://facebook.stackoverflow.com/, that was due to a tieup with Facebook (or something like that). And I don't see how it helps. Tag favoriting and all already get the job done efficiently, IIRC the facebook.SO site isn't used by many.
What would a subdomain solve that isn't already provided by the tag ruby-on-rails? I don't think that programmers would be confused about the existence of other programming lanuages and frameworks on Stack Overflow, so I see no need for a subdomain.
The Facebook subsite was an experiment, and I haven't heard any positive comments about it. So I strongly doubt that SE will add any more subsites to SE.