When I'm on facebook.stackoverflow.com and click on the 'meta' link in the header, I'm taken to the meta site. But when I then click on the 'main' link in the header I'm taken to Stack Overflow proper instead back to the facebook. subdomain. From meta, I cannot find a way to link back to the mini-site.
I'm sure this would be a PITA to implement properly, but I just thought I would bring it up as a possible source of UX confusion in case there was in fact something that could be done to prevent this confusion.