The technology behind the Oculus Rift was developed on Zenimax's computers, and on Zenimax's time, says the parent company of ID Software. John Carmack, who went to work for Oculus, therefore is not the owner of the patent of the operating system by law. It falls under his contract at the time with Zenimax, and thus they are claiming at least a portion or compensation before the sale of the Oculus Rift and Facebook goes through. This is similar to a chemist developing something like Teflon when working for Dow. By law, the patent belongs to the person who owned the equipment, and on whose time it was developed.
Whether this means a delay, or nullification, of the sale of Oculus Rift is yet to be seen. The initial notice has been sent, which is the prelude to a court challenge.