There's apparently been something like 100 million downloads at least (actually that number is quite old -- from when there used to be about 2.5 million online users at any one time). If that translates to 100 million registered users these days, only Skype knows. I can easily believe that there must be at least four or five times as many registered users as there are online users. I know some people that are online only for a few days at the time now and then, and even at work where we actively use Skype there are only about 10% online as of this moment (holidays etc., but even when everybody's here there's about 1/2 - 2/3 online at the same time). That the number of online users just keep growing tells much I think.