Market Capitalization (market cap) is a term from finance which refers to the number of shares of your company outstanding multiplied by the average dollar value of each share - http://en.wikipedia.org/wiki/Market_capitalization . I think what you are after is potential market size, not market cap.

Anyways, one issue with your calculation is that you assume that 100% of people have the ability to use your service - I assume they need a computer, Internet access + a credit card of some type to use your service, and 100% of US full time works do not have all three those things.