Compañeros/as, desde WordPress me han enviado un mail con respecto a este problema, lo copio aquí para que lo puedan leer todos, me dicen el problema que tuvo i la solución a el mismo.
Thanks for trying out our new Facebook: Publicize feature.
Sadly, we've found a bug in the feature that may prevent your WordPress.com posts from getting sent to Facebook correctly.
We've fixed the bug, but since you're an early adopter, you're still suffering the consequences.
Happily, there's an easy fix. All you have to do is go through the WordPress.com <-> Facebook authorization again.
Just go back to your Dashboard -> My Blogs admin page, uncheck the Facebook checkbox, wait a couple seconds, then recheck the checkbox.
You'll see a message asking you to authorize the connection between WordPress.com and Facebook; click the button and you're on your way.
If you need more help, please contact us at http://support.wordpress.com/
Sorry for the Hassle, but Happy Blogging on WordPress.com!