Microsoft recently made some exciting changes to SkyDrive.  Before when you shared a document and allowed users to edit they were required to have a Microsoft account.  Now when you share a document with edit rights they don’t need a Microsoft account.  If you require them to have a Microsoft account you can make that mandatory when sharing documents.  I would also like to point out that multiple people can still edit the same document at the same time. 

As for numbers today there are over 1 billion Office documents on SkyDrive.