I don't think it should be a joke at all. It seems to me that a company that doesn't know what its business plan is should probably be hiring people who have a talent for making business plans.
As it is, my own guess is that facebook's business plan involves selling the personal information of its members. This would be the most obvious commodity and also the most obvious reason why facebook wouldn't want to talk about their business plans.
For example, with Facebook Connect, it's much harder to be an anonymous troll commenter on a blog. And maybe using Facebook Connect as a login for eCommerce sites would reduce the amount of fraud those sites have to deal with. That is definitely monetizeable.