Concurrent Skype Users Online at 18 GMT today: 19,142,581 . It is a new record, and it has been only 21 days since 18 million.
This may seem to be very fast growth, but let's note that the growth rate is more subdued in the middle of the European day and in the middle of the East Asian day. Skype is pumping out the fabulous numbers only in time slots corresponding to the Americas. For sure, the rest of the Skype world is growing fast, but just not as fast. No matter where or how you measure it, it's exponential since at least 2007.
Prediction: 20 million online at 18 GMT on Monday November 30th, about 3 weeks before Christmas!!