Scuttlebutt [1] is the Decentralized Mesh you're looking for. Still early stages but will only improve over time. Has a few thousand users (approx, it's a mesh after all), and is developed completely in the open with clients written in multiple languages.

It can't be censored or shut down because all communication spreads through peers, with few central points that can be blocked.

1. https://scuttlebutt.nz