In the next few days, Skype will pass the 500 million downloads mark. I seem to remember someone saying that Kazaa was the most downloaded program ever, and looking at the Kazaa home page, they currently show 389 Million. Skype is at 498 Million as I write this, and is downloading at almost a million a day. The download rate was over 1M a day when they made a new release available and existing users were getting updates. Compared to Kazaa, Skype is downloading every day, what Kazaa is downloading every week.
How are the competition doing? Vonage added a few hundred thousand users last quarter. Thats a few orders of magnitude away from being a competitor!
Of course, Kazaa is based on an earlier version of the P2P technology that powers Skype, and which is about to come to the world again via Joost. Joost just released a new beta for Windows, and their first beta for Intel based OSX. I still can't justify upgrading from my own Powerbook G4 (it just works) so I'll have to wait for them to backport to the older systems, and use my work laptop.