This is unbelievable. Skype is blaming Microsoft "Patch Tuesday" for the downtime and the sign-on problems that lasted for more than 48 hours.
As per Skype, the disruption was initiated by a massive restart of our user's computers across the globe within a very short timeframe as they re-booted after receiving a routine software update. This is presumably the Windows updates that were sent out on Patch Tuesday last week, which required the PC to be restarted.
Normally Skype’s peer-to-peer network has an inbuilt ability to self-heal, however, this event revealed a previously unseen software bug within the network resource allocation algorithm which prevented the self-healing function from working quickly. Regrettably, as a result of this disruption, Skype was unavailable to the majority of its users for approximately two days.
It's kind of funny because Skype is using Windows Update as an excuse rather than admitting that they had a bug in their software that was exposed when Microsoft fixed their Windows software. Patch Tuesdays happen every month - how could Skype survive the previous "bulk restarts" on Windows machines.
Update: Abhishek has the gist of the whole story:
In brief what happened was that skype had architected its applications in a particular fashion with a known but uneventful bug. Once the patch was applied, that bug in skype server application became active and prevented recovery of the network!