Neither side is 'kindly donating' their portion. Both are getting back more than they give, in a voluntary mutually-beneficial transaction. Or they'd each do something else instead.
Now and then I see someone complain about a free service, Google for instance, and receive a response such as "Well you're entitled to all your money back". This is stated as if the customer has no right to complain, because the service is 'free'. As you noted, that it is not the case. It's more of a symbiotic relationship.
Thought Facebook is clearly free to do as they wish, I personally feel that using my messages to friends to promote products for Facebook's profit is something other than what I expected when I signed up.