What usually happens is that an established local or online business or organization creates Facebook pages, Twitter feeds and the like. Sooner or later they realize the immense benefits that these can bestow and so put more time and energy into their presence on these networks. Eventually they have teeming accounts that reach thousands of new prospects. They then become local social media standouts. Examples of this approach include Perth Zoo, Fremantle's Esplanade Hotel and Chalkys Espresso Bar.
But so powerful and pervasive is social media that you can actually do it the other way around as well. One example of this is the dominant city-centric Twitter account Tweet Perth, managed by Adam Barrell. This has long had a huge amount of influence. I know this from personal experience. A couple of years ago now he retweeted a blog post from my local blog about this city. I was pleasantly surprised to see that I'd received about hundred clicks in an hour as a result. No wonder so many local businesses hope for this kind of attention!
Barrell has used that powerful online presence as a springboard to create a whole new local content platform. I suspect there are other, bigger projects in the pipeline too.
Lost Perth is another case in point. This started out as a Facebook page that encouraged people to share their local reminiscences. It quickly took on a life of its own and became hugely active and popular. It even inspired a couple of TV documentaries. A website resulted that sells merchandise such as t-shirts and a book inspired by the page. Not sure how much money the whole project has made its creators, but I suspect it's a tidy amount by now.
So, if you've got a social media presence that is solid and growing, there's no reason you can't make it into something profitable and full-time by branching out in bigger ways both online and off.