I've grown https://www.uncover.com (a simple tool to give employees perks and rewards) in various different ways. A lot of it began with telling my network of friends who run startups. Getting them signed up. Then getting them to tell their friends how much they liked it. Once that source was depleted, I began to do a lot of content marketing. I started writing for a lot of different blogs, websites, etc. That helped get out name out there and brought in about a third of our current customers. I'm now beginning to experiment with buying ads. It's still too early to tell how well that will work out, though.