Not sure what you're referring to, re: 10 million units. They haven't sold anywhere near that.
Currently their price to sales ratio is closer to 150. Facebook paid a massive premium no matter how you cut it. Even if they grow sales ten fold over the next three years, they'll still have paid a premium. I don't think the Oculus purchase can be justified in any near-term financial regard, it's a very long term bet.
"Oculus has said it will start selling a consumer product in 2014, and till now has shipped an impressive 60,000 units to developers at $350 each"
http://www.forbes.com/sites/parmyolson/2014/03/25/facebooks-...