Some of the standard themes on WordPress.com support infite scroll, meaning additional posts are loaded and displayed as you approach the bottom of the page. Our metrics have shown that this increases reader retention. You can see it in action at http://raanan.com or http://matt.wordpress.com.
If you would like to add this functionality to your theme, please visit the Jetpack support page on Infinite Scroll.
Note: when you enable Infinite Scroll, the posts per page value is changed to seven. Every Ajax load puts seven more posts and counts as one pageview.