VoluntaryNet is an in-browser webrtc-based decentralized messaging platform on which social networks, marketplaces, and other applications can be built. Unlike centralized or federated systems, you own your identity and data. It's free, open source, and run by the users themselves.
Check out the ALPHA release:

demo app   and   source code

Apologies for the current lack of documentation.
Release announcements posted on @voluntarynet.