Check out the secure scuttlebut protocol. Very straightforward messaging via LAN.
Scuttlebot forms a global cryptographic social network with its peers. Each user is identified by a public key, and publishes a log of signed messages, which other users follow socially.
Scuttlebot searches the P2P mesh for new messages and files from followed users and from FoaFs (friend of a friend's). The messages and files are stored locally, indefinitely, for applications to read.
DELETE FACEBOOK