I believe the point is less about revenue being a guarantee that it won't happen and more about the fact that a lack of profit makes it increasingly likely that the product will eventually be shuttered because free isn't a sustainable model.
I doubt Friendfeed makes Facebook any money, and was clearly a talent acquisition (Bret is their CTO now) but they still keep it running.
You're right that anyone using any tool/service with an unsustainable model needs to be cautious. But there's a lot more to it then is being proposed by the OP.
Bret Taylor (ex FF) is Facebook CTO, and clearly wants FF to keep running. Since it's cheap, who's going to tell him "no"?