Even if you own a totally online business, all of your marketing efforts don't have to be online. Get out there in your community, and interact with all of those potential customers! People often think of online companies as faceless entities that communicate only through email and Twitter, so you can set yourself apart from the crowd by mixing and mingling. If people can connect a face with your business, they might be more likely to buy from you than from competitors they've never met.
Of course, you'll also be gleaning information from everyone you contact while you're out there in the world. The key is to transform these connections into an online form. If you're selling your goods at a neighborhood festival, be sure that customers take advantage of the handy email sign-up sheet in your booth. Collect business cards at a networking event, and be sure to add those names to your email list and start following everyone on social media. Then you can try to make these new friends into loyal customers.