Larry and Sergey's PageRank isn't a new application level idea (as search engines already existed) but rather a better implementation of an existing application level idea.
"@Jack was still just an engineer, and the service was only a few months old when the group acquired Twitter.com and re-branded. Back then, we had no character limit on our system. Messages longer than 160 characters (the common SMS carrier limit) were split into multiple texts and delivered (somewhat) sequentially. There were other bugs, and a mounting SMS bill. The team decided to place a limit on the number of characters that would go out via SMS for each post. They settled on 140, in order to leave room for the username and the colon in front of the message."
http://www.140characters.com/2009/01/30/how-twitter-was-born...
Perhaps you could say that twitter is an accidental success that is much more interesting than the original vision.