Free P2P alternative to Twitter: Twister
Twister is a microblogging platform based on the P2P principles from Bitcoin and Bittorrent. It is a scalable platform for censor-resistant posting and private messaging, with end-to-end encryption. Quoting slashdot:
The basic concepts are described in FAQ while more in-depth technical details are available from the white paper. The twister network is running already: the client can be compiled for Linux, Mac, and Android. 2500 usernames were registered in the first 6 days.