The former founders of Kazaa today launched Rdio (link ready soon), their first fully legal music service. Like some of its competitors, it promises per-track streaming both on the web and on mobile devices like the BlackBerry and iPhone for a fee. Rdio promises to break from the conventional formula by letting fellow users share word of what they're listening to, see the top songs on the network in a "heavy rotation" list, and make tracks public on Facebook or Twitter.
