When it comes to social media, there's more to life than what I call the "Royal Three" especially for finding business to business leads.
Your first thought should be, find other places where your audience hangs out online:
If you're having difficulty finding "niche" sites, try searching for "niche social sites" plus whatever industry you're in. You could also do some research to see what your competitors are doing. Once you've found sites to contribute to, you can start commenting on blogs, answering questions, joining a discussion, adding events to a calendar, or any other social activity available on the site. For example, I'm getting traction with startups on Quora, a Q&A type social site.
Many of these sites allow you to add your name to a directory as well. I wouldn't ordinarily recommend paying for a directory listing unless you start seeing a fair amount of traffic from it. This idea is applicable to the social activities as well. If you start getting traction, you could consider advertising or paid sponsorship and closely track the results.
The benefits of this social strategy include:
Anything you do socially will be a longer term play and take more effort than advertising. Paid sponsorship may speed up the process. No matter what your approach, you'll need to determine whether the time and financial investment is warranted in terms of results you're getting from other promotional tactics.