Email's federated nature has kept it alive for decades, and will ensure that it long outlives more proprietary forms of social networking.
Federated systems already work in practice and at scale.
edit: Some other examples with widespread adoption:
- The phone network (PSTN)
- The internet (via BGP)
- The postal service (federated between countries via the UPU)
- Podcasts (and RSS more generally)
Am I missing any?