FireChat is a free app for iOs and Android that bills itself as an "off the grid" form of texting. It can work online or without an internet connection. In its "nearby" mode, the app uses bluetooth to connect to other mobiles close to it to send messages between phones. A FireChat discussion can include as many as 10,000 people. It has been downloaded by more than 100,000 protesters in Hong Kong, to let protesters strategize without internet connectivity. The app is more like a chat room than an instant messaging platform, used to send mass notices about police activity amongst protesters.
However the information on the app is not encrypted so users concerned about privacy should not use their real names.