Publicize to Twitter and Facebook

WordPress.com sends your published blog posts to several places to automatically promote them for you. For example, your posts end up in WordPress.com’s global tag pages, are sent out to many search engines via Ping-o-Matic!, and sitemaps pings notify search engines of new updates.

In addition to these built in features, you can also choose to send your posts to any of several third party services. Currently, WordPress.com Publicize supports Facebook, Twitter and Yahoo!.

Find out how it is done:  http://en.support.wordpress.com/publicize/