This is known by most Facebook App Developers.

What makes up the number shown on my Like button? http://developers.facebook.com/docs/reference/plugins/like/

The number shown is the sum of:

The number of likes of this URL. The number of shares of this URL (this includes copy/pasting a link back to Facebook). The number of likes and comments on stories on Facebook about this URL. The number of inbox messages containing this URL as an attachment.