Hi PhotoGazza,
We pull Facebook Shares and Facebook Likes from Facebook's FQL. They define Shares as, "The number of times users have shared the page on Facebook." and Likes as, "The number of times Facebook users have 'Liked' the page, or liked any comments or re-shares of this page." In this context, a page is defined as a URL on the web (ie not a Facebook page).
So, I believe that a share is a count of the total number of times that Facebook knows a URL has been shared on Facebook in general (whether through a status update, comment, etc, public and private) but to be perfectly honest, Facebook doesn't make it 100% clear. You can check out their documentation here: http://developers.facebook.com/docs/reference/fql/link_stat/
Hope that helps. Let me know if you have any more questions. Thanks!
Miranda