This is from WordPress.com Support: http://en.support.wordpress.com/publicize/
Sometimes, the connection between Facebook and WordPress.com might get disconnected. Not to worry, the following steps should get you up and running again quickly.
- Log into your WordPress.com blog. Go to your blog’s dashboard -> Settings -> Sharing and click the Disconnect from Facebook link.
- Reload the page and confirm the link now says Reconnect to Facebook.
- Log into your Facebook account. Click on the arrow next to Home on the upper-right side of the page. Then, go to Privacy Settings and then click ‘Edit your settings’ under ‘Apps and Websites’ at the bottom-left of the page. Remove WordPress.com by clicking the X in its row. (If there is no WordPress.com row, don’t worry, just skip this step).
- Log back into your WordPress.com blog. Go to your blog’s dashboard -> Settings -> Sharing and follow these steps to reconnect.