In addition to these sites, start volunteering for local/nearby charities. You'll find that it's a good way to build up your network (why do you think so many execs are on nonprofit boards?) and your credibility within the nonprofit space.
We blog for a number of reasons. Since we run a B2B business, we'll write about topics of interest (i.e. issues in the B2B or software space), our community/users and the company, as appropriate.
Go to Google. Search for your favorite companies. See if they have Google Places pages, Facebook pages or websites. Chances are some of the smaller ones won't have the latter.
We have a few contributors for the G2 Crowd blog (http://about.g2crowd.com/blog), but it's a great place not only for sharing company news, POVs, media coverage and more, but to publish highlights from your user/customer base as well.
This is critical. As a marketer, I've seen - far too often, mind you - a severe disconnect between the perception of one's customers, and the reality.
Not saying that you need to have full personas, in which you include ALL motivations, demographics, customer journeys - the works - but it helps to have something on paper that you can refer to.
Good point. They don't stay long, and they likely won't sign up for anything. If you get a big spike from SU visits, it could take down the site. And you'll have nothing to show for it.
That's a great point. So many sites - including TC/VB - are the same folks all the time. And they might be keeping up with startup news, but they aren't potential end users.
For business-related products, it's also helpful to gain exposure in the industries you're in. For instance, if you're launching a product that might serve both the healthcare and legal sectors, any type of exposure there not only proves legitimacy for potential customers, but to investors as well.
Unless they can somehow seed it with a bunch of images, get some decent answers and go from there. (Which is what they're going for eventually, I think.)