facebook does not offer a way to change the description, but you can set the tiitle.
in href portion of the like code:
href="http://www.facebook.com/sharer.php?u=<?php echo urlencode('http://yourdomain.co/page.php'); ?>"
you can add a t url variable to pass the title:
href="http://www.facebook.com/sharer.php?u=<?php echo urlencode('http://yourdomain.co/page.php&t=your title here'); ?>"
facebook share documentation can be found here:
share/