The link usually is something like
http://www.yourdomain.com/activation...d=realusername
In some cases, the "realusername" id replaced by a strange string, as the one you posted above. This usually happens when the password of the member is encrypted (in example using md5()) so the activation page recieves the $_GET value for the password and performs a query in the database for the "password" field matching the string.
I prefer to only use the member username, as it doesn't create security issues and simple enough.