Every web developer should know about this - if content is worth sharing, it's worth sharing how you want it
To get web page details shared on facebook how you want them...being able to choose which image is used, what the title is, the description, even the canonical url...you use the facebook open graph api
We've just recently utilized this, and thought it was worth sharing.
Just a bit of theme work, in a content management system (drupal page.tpl.php or html.tpl.php), and each of your pages can be shared more effectively.
We're looking forward to more social integration across the web, and contributing to it.