Paired up with the popular "like" button, the new "send" button allows you to send a web page to a select list of your fb friends. So, it's similar to an "email this" button, but it's connected just to your list of facebook friends. That's the marketing potential behind it: the power of word-of-mouth recommendation.
Just as when you send a message from within fb, it will intelligently complete names as you begin to type in the recipients. Once you place the code on your site (if you use the XFBML code), all previous likes will be computed and displayed. Also, the first person who clicks on the send button, once it's installed on your web page, will have a thumbnail of their fb profile pic displayed and their name. The next person to like it from your web page, will have their photo displayed, and name. etc.
Go here to get the facebook like + send buttons.
Note: If you don't want faces to be shown of "likers" then in the code (show_faces="true") change "true" to "false" (show_faces="false"). Also, experiment with placement. While "like" buttons often appear at the top of an article/video/other item, this strip of code (like button + send button + how many friends have liked it so far) makes the loading of pages on my drupal site go through one additional "click/hiccup"--at least in IE.