Just out of curiosity. How did this url fragment #.TfMwNJgETxs;hackernews end up in the URL? How did ReadWriteWeb know that this URL would be posted on Hacker N

Just out of curiosity. How did this url fragment #.TfMwNJgETxs;hackernews end up in the URL? How did ReadWriteWeb know that this URL would be posted on Hacker News? Edit: Oh, I found out by myself. There's a link on the page to post the story on Hacker News. So it seems like the url fragment was added by http://www.addthis.com http://www.addthis.com