The industry lacks an open social network. A social network can be build on top of email: the user interface can be a specialised email client with email as the transport mechanism. Everybody has an email address and web frameworks can easily communicate using mail protocols.
Email can be used as a common denominator protocol. A flexible and federated social network would abstract the protocol used for communication and upgrade to a better or efficient protocol when appropriate such as realtime applications or when sharing files.
Users can send invitation requests in a regular email that includes a public key.
All shared content remains stored on the user's computer but can be hosted elsewhere.
Diaspora, BuddyCloud, Status.Net, SockNet, Tent, pump.io