Facebook - How to add a share button

As of June 2012, you can use a simple link that follows this form. 

Of course, the link can be added to your page as simple text or be offered through a graphical icon.

http://www.facebook.com/sharer/sharer.php?v=4&src=bm&u=http%3A%2F%2FDOMAIN.COM&t=TITLE