Of course, not every site has to be a general consumer site to generate revenue. It's very possible that your online business idea is focused more on selling to companies or business people. For example, maybe your site provides access to venture capitalists and angel investors that small business owners looking for investment funds may want to reach out to. If this is the case, you should look at LinkedIn as a primary resource for marketing. Once thought of as a way for job seekers to find employment, LinkedIn has evolved into the premier online source for business professionals to interact with each other.
There are many things you can do with LinkedIn, including leaving status updates like you might leave on Facebook (although usually with more of a business edge rather than vacation pictures or updates on what you're having for breakfast). You can also post links back to your site or articles you think your network would find interesting. But the best way to use LinkedIn for your marketing is to get involved in groups. To access groups, click the Groups link at the top of the home page and choose Group Directory. From there, you can use keywords to find groups in your industry. These groups are composed of people interested in the same topic, business, or industry as you are, who congregate online to discuss related topics.
Join as many groups as you can (that make sense for your business) and jump in on the conversations. Set yourself up as an expert so that over time, people within the group will come to trust your opinions, connect with you, and visit your site. Depending on the group, you may also be able to start a conversation of your own, drawing more attention to you and your site. For example, a recent blog post that you wrote could be good fodder for a new group conversation.
Better yet, start your own group. It might take some time, but it could prove worthwhile. Not only do you get to control the messages within the group, but you'll have access to all members to send them in-site e-mails. This gives you a significant marketing advantage when you want to promote something particularly exciting.