Answer: Facebook’s purpose is to perpetuate Facebook. Forget all the crap about connecting people, etc. It exists simply to perpetuate itself, like a malign organism.
This thought was triggered by a terrific essay by Paul Ford on the essence of Facebook, as revealed by its Android cuckoo, Facebook Home. Finishes with this memorable summary:
The moral vision of the Dynabook posited that people would use technology to manipulate code and data, to create models of the world—as many as they needed in order to understand it. In contrast, Facebook has a single model of the world, unapologetically monolithic: the canonical graph of the relationships between more than a billion human beings. If the company is to grow, it must insert itself between people and their smartphones; there are still simply too many moments spent watching things, or reading things, or making things, that it does not own.