@timetheif is correct that WordPress.com blogs are not able to do advertising, which is really a bummer and one reason I am considering moving to a self-hosted platform in the near future.
But anyway, to answer the initial question, you have to be active in the WP community and comment and interact with other bloggers. To me that is the best way to attract more readers. Even pressing the Like button sends an email to the author with links to some of your recent posts - so if you have interesting titles, they may decide to click on them.
Social media can work but takes a LOT of time and effort. You can certainly publish your posts to your Facebook page, but after a while you're friends will probably ignore you. Twitter is a waste of time too unless you are going to take a lot of time to cultivate relationships with others. I find most people on Twitter just want to promote their music or send you spam about winning a free iPad.
And I don't care much for other social sites like Reddit, Digg, and StumbleUpon because I don't like the fact that they frown upon self promotion.