I'm pretty sure Yelp has actual usage data that provides a better signal of which cities are worth the effort than population. Plus, trendy cities get talked about more than non-trendy ones by definition. More people care where hipsters hang out in Austin than where they hang out in Phoenix even though Austin is pretty small in the grand scheme of things.
The problem is, in not including the top 10 most populated cities in the US in things like this, they exclude a lot of the nation. This example in particular may not be great... but I am constantly seeing posts about new service X that doesn't include even the top 5 most populated cities... It's like excluding a large part of potential users. It's similar to releasing an IE-only site today.
