No matter how many customers you have use them as potential visitors to your site. The larger the customer base the better. We are dealing with tens of millions of people already. In the next few years it will be in the hundreds of millions. Probably only a small fraction.
Niche marketing is the way to go.
Focus on an area, and you will thrive. Most people take e-mail for granted, it is an extremely valuable tool. It means use the power of e-mail to your advantage. I have e-mailed and received responses from major publications and high level management personnel. I know via other communications I would not have gotten a response.