"Rel=”nofollow” is an attribute that webmasters can add within the code for external links in order to signal that the link should not be followed by the search engine spiders. Bloggers often use the attribute to disuade comment spam. The nofollow trend on Twitter started last year when they first indicated to search spiders that links posted under Web, on the right hand side of the Twitter profile, were not to be trusted. They then extended that to links within the Bio as well. Now, Twitter adds the rel=”nofollow” attribute to all external links, be it links referencing an article or links to third-party Twitter apps that enabled posting the tweet. Internal links remain untouched, turning Twitter into a walled garden much like Wikipedia, with lots of link juice coming in from other sites but none going out." - Susan Beebe