Presumably to save on server load, as soon as you add a share button on your site and hit the link the description and image thumbnails are cached. 

 

This can leave you with place holder copy or out of date content and there is little or no information on the (shamefully poor) Facebook documentation. 

 

To get round this and clear the cache add a query string in the share link for example:

<a href="http://www.facebook.com/share.php?u=http://blog.grahamlicence.co.uk/?123" title="Share on Facebook">Facebook</a>