I've just committed two new plugins to the habari-extras Subversion. Firstly, there's the 'share' plugin, which adds the Facebook OpenGraph metadata to pages so that the Like button works, and also adds Facebook and Twitter widgets to the bottom of posts. Secondly, there's a bitly plugin, that pings bit.ly's API when every post is published and generates a short URL. This URL is then stored in the post info, and it provides a template to add it to the head of each page using the shortlink markup. I'd be interested in feedback from anyone more familar with Habari in case there's a better way to do what I'm doing.