Skype says it has restored service to around 10 million customers, leaving up to 15 million more still unable to sign in.

The company notes that even those who do get back online may find that it takes time after signing to show up as “online”, and instant messaging might not be quite so instant.

Ominously the company also notes that “Group video calling will take longer to return to normal.” Given that we’re just two days away from what must surely be the point of the year that has the most demand for that service, it’s safe to say Skype bosses won’t be having a very merry Christmas this year.

It’s not possible to get a precise figure of how many users can and can’t access the service as Skype doesn’t run central servers — instead it’s a torrent-like peer-to-peer service which uses the processing power of computers that are signed in. (And if you’ve ever thought your computer runs slower while signed in, you probably aren’t imagining things.)

It’s this system that appears to be the cause of the problems. By default, any computer signed in to Skype is configured as a “supernode” in the network. Though this doesn’t mean users’ computers route call data, they do act as a key point in making sure one Skype user’s computer is able to find and connect with another’s. But this system has failed yesterday and is only gradually coming back online. The outage is already the longest in Skype’s history.

Engineers are now working on manually setting up what Skype beautifully names “mega-supernodes.”

The company’s commercial business service, Skype Enterprise, has not been affected by the problems.