Seems I stand corrected on the mobile front.
But having had some more time to think about it, as Tuscani mentioned earlier, the hyperlocal focus of UT has always been one of the major draws. Interrelated to that, I'd add that another key aspect of UT's appeal has been the fact that there's always been a certain presence of individuals who, to varying extents, are involved in urban development in some capacity (Adma, UrbanShocker, ProjectEnd, or Skyjacked, as random examples), and their contributions have been absolutely invaluable in providing a level of insight and basis for intelligent conversation that is lacking on some of what I'd now define as competing sites (SSC in particular).
So for the different communities to truly thrive, I think there has to be a concerted effort on SRC's part not simply to attract casual skypscraper fanboys, but to proactively also try and get folks who are similarly involved in their respective cities to become contributors, providing a backbone of sorts for these incipient communities, creating value for other new members, who are similarly passionate about their city, to join, and going beyond the simple photo/project updates, fostering a much more nuanced and discourse based platform for discussing urban development, and everything that goes with it (incidentally, the guys behind
this website may be good such candidates for Vancouver, as they're already providing regular and frequent development updates in Vancouver on their website, though it lacks a proper discussion platform). I also recall hearing it remarked that there are still certain developers and architects in Toronto who don't 'get,' UT, so there also needs to be some ability for educating these possible contributors on precisely why they should get involved as well.
Realistically, I have a hard time seeing UT successfully scale out to provide truly comparable communities worldwide with SRC, and I'd rather have seen a much more targeted focus on growth, as WislaHD mentioned, possibly with an UrbanCalgary, UrbanMontreal, or UrbanVancouver, which I think would also serve as better and much more relevant base points for conversations that tie back in with those here.