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.