After a daylong outage that left untold millions of Skype users high and dry, the CEO of the peer-to-peer chat service apologized, promised that service is slowly but surely being restored, and announced that credits may be in the offing for paying users. The root cause of Wednesday's massive Skype failure is still a mystery, however.

In a blog post and in a just-posted YouTube video, Skype CEO Tony Bates said that as of Thursday morning, about 16.5 million users were back online — still only about 80 percent of the number you'd normally expect to see using Skype on a given weekday morning, but greatly improved from Wednesday, when as few as 1.6 million users were connected.

Bates said that Skype's "core services," such as audio, video and text chat, had been "stabilized" after engineers rejiggered a series of servers to act as "supernodes," or Skype clients that help one Skype user locate and connect to another.

That's the good news. The bad news is that Skype engineers ended up repurposing some servers used for other Skype features — namely group video chat and offline instant messaging — to shore up the "core" video and audio chat services. For now, that means anyone hoping to engage in group video chat or send offline IMs is out of luck.

Bates then apologized for the meltdown, the worst at Skype since 2007, when Skype users were left without Skype chat for two days. That outage was later traced to a "massive" number of Skype-connected PCs rebooting in unison after Microsoft issued an unusually large number of Windows patches at once.