WP LikeJS includes a nice jQuery driven Like Box Popup Box onto your Website or Blog, which pops up when the user is scrolling your page down. You can choose how much px have to be scrolled before the LikeJS Box is displayed.
Also you can change the headline of the Box for your communication with your visitors like "Follow us on Facebook" or whatever you want.
The script is very smart and loads fast and is compatible with all mayor browsers.
Demo:
http://www.like-js.de