For the sake of replying because barely anyone ever replies to these kinds of posts, I will explain how to do this after digging around for 2 hours and finally figuring it out.
A plugin that will allow you to do this is "End Page Slide Box" and it's available in the WP plugin repository.
I used another plugin called "WP-Insert" that allowed me to place the code that you need to put into all posts for your Facebook like box to appear when the user scrolls to the bottom of the page.
This is the code:
[EPSB]Facebook iFrame goes here[/EPSB]
Once you save that in WP-Insert, you got yourself the same thing that ViralNova has, and it even looks more slick. You might need to add some text to the top of the slider so it says "We post stuff just like this every day on Facebook. Like us. You won't regret it.".
Hope that helps :).