Well, orkut is a product of Google. Facebook has millions of users whereas orkut does not have that much users compare to facebook. Plus facebook got various good applications and games.
I can just differenciate by saying that facebook is a service used globally, however orkut has the most number of users only in asia.
The difference between Orkut and Facebook is Orkut is Google's Social Networking is place to keep in touch with your friend, Facebook in general is place to find your schoolmate/college. Facebook also have a lot of applications, Orkut doesn't have.