Hubski isn't really all that empty, it just goes at a much more leisurely pace. Part of the reason is that we don't have as many users, yes, but also part of that is because our users are spread all over the world. We have regular posters from Australia/New Zealand, Russia, France, those Scandanavian countries I can never keep straight (sorry veen and Devac!), and the U.S. of course has four time zones and everyone keeps different professional hours. When you've been here for a minute or two, you quickly get used to the idea of posting a comment and then checking back half a day later to see if you got a response.
As for concentrating the community, activity here is so light, you can see everything going on in Global and Chatter in a single day without much effort. One of the things I do tend to recommend people though, cause I see that it helps, is to keep tags a bit on the vague side. For example, if I'm talking about a race, I'll use #cars instead of #f1 or #dragracing because the likely hood of people following the more specific tags is pretty low. The post is gonna be visible in Global anyway, because there's not much it's competing at, but this way it's easier for people to look back and find old posts under certain subjects.
Edit: Also, thread necromancy here is both allowed and even somewhat encouraged. So don't be afraid to comment on a thread that's 2 months or even 2 years old.