It's the chicken or the egg question and in all truth you need both to grow each other. You need to figure out about the minimum level of businesses you need to keep customers coming back and back. Second, you need to offer those business a free trial period, say 3-6 months, so that you have the businesses that will attract the people, but the businesses are not throwing away their money on an empty project, while you are recruiting your people.

You also need to market yourself and start early, while you are still setting things up. Get the word out: make youtube videos. Ask other sites to attach a link to your site. Write on blogs. Leave links on blogs (it is a bit of spamming but if the blog owners don't like it, they'll delete the link). Even better if you can do a trade for service with those same blog owners.

Be creative and MARKET MARKET MARKET. There's an old Russian saying: not advertising your business is like flirting with a girl across a non-lit room.