From the ReadWriteWeb case study:

“I used to get hit by Stumbleupon and [the site] would start to crawl. Then I changed to some of this real time stuff and I’ve reduced the number of servers. Instead of the users sitting on the page and refreshing, I push it out to them. My EC2 bill has gone way down.” Roden’s experience compliments the story that Google’s Brad Fitzpatrick told us about using PubSubHubbub push feeds to deliver shared items in Google Reader to FriendFeed. Changing from polling to real-time push cut traffic between the two sites by 85%. Likewise, magazine-style feed reader Feedly says that the part of its service that now consumes PubSubHubbub from Google Reader has seen a 72% reduction in bandwidth.

Read the rest of the case study here.