Build distribution into your product. Add a little link somewhere "XYZ Functionality powered by XYZ Product". Other business owners will see it and sign up. Make it tasteful and small so it doesn't annoy people.
I am also writing to a couple of popular blogs in industries I have chosen to start in, to give it away free as an experiment. It is in the hope that the "powered by" link could get me other users in that industry interested.