I run a service for bloggers. It's a bit of both: online directory and blogging community.
I wonder how can I target that group of audience most effectively and at the lowest possible cost. I've been writing on technorati, del.icio.us, and I do have several signups from there. However I'm not that satisfied with those results.
post a video on you tube (a review of the thing that you promote) add the site to social bookmarking sites (stumble upon, spicy page etc.) attach a blog to your site and add it to blog directories. start a contest on your blog and give the winner a tempting prize invite a guest blogger to blog about a topic on your blog get a review from the review me site
There are loads of ways to spread the buzz. Do not hesitate to utilize them all. And don't be scared of getting badmouthing about your site. The do get attention as well. Hope you find other ways and good luck!
If you host blogs and want people to use your service (if it's not hosting blogs or make people sign up for one - skip the rest of this) - then why would you want to get in contact with people who already have a blog? I would target people who doesn't have one, and make them realize how much they just need it and offer them my super duper service because i'm so nice to make them realize how much they just need it.. that's what I would do anyways, but i've been wrong before - and might also have misunderstood what kind of service it is you actually have..