As a directory, your main target audience are small / medium / large businesses. Your marketing needs to be very specific to them, perhaps you can try some of the following ideas:
- reach out to various associations for specific trades eg plumbers, painters, electricians etc and offer to advertise your directory listing service with them
- create a Facebook campaign targetting users who have interests listed around small business. You may want to list out 30 or so industries of small businesses who are suitable to list in your directory and use those keywords in your Facebook campaign to attract them specifically
- write excellent quality content on your website blog relating to topics that all your users would find useful to attract relevant people looking for this information.
There is a website in Australia HiPages which is a directory for specific trade businesses and they do this very well - I would direct you to study their articles to see how they specifically attract interest by industry on their topics, which ultimately result in directory listings on their site.