Cause related social marketing friend building

I have a cause related Facebook page the is relevant to US motorists and would like to spike people who joined my cause. It is going well. This is a cause that is relevant to motorists and tourists in the eastern United States particularly and truck drivers nationwide. While my goal is to get my numbers up, I am interested in educating recruiting people who have relevant interest in the cause...getting 300 people from another continent to join, for example, would actually be counter productive. The focus needs to be on folks from the US. While I am open to any process, my advice would be to find and post to relevant Usenet groups...I can provide a sample message. Please provide general methodology and time frame as well as cost and confirm that you understand that the primary goal is to recruit motorists from the eastern United States.