I see at least two ways that having data in Facebook has the potential to help governments learn more about people: things (relationships, events, biographical data) are pre-structured in a single clear ontology, and Facebook has direct access to all of the facts in the system.

If we imagine a more decentralized system, people might well still adopt a single ontology for events and relationships, but much "private" information might actually never be revealed to third parties, as opposed to disclosing all of it to Facebook and asking Facebook to limit whom they show it to.