alt Hacker News> definitely not practical for messaging
Text based messaging ala IRC? Just how quickly and how much do you type? A few hundred KiB exchanged between nodes only every 10 seconds or so ought to be able to accommodate thousands of simultaneous users in most scenarios. The impact on battery life should be far less than using a bluetooth headset.
Sorry I should be clearer: I think it actually might be feasible in a high population density area and if everyone uses it, but because of the limited range of bluetooth you really do need a high density of active nodes for it to work reliably.
A messaging system that often takes hours or days to get messages to the receiver is fairly useless and people will continue to prefer centralised systems, so there's a severe chicken-and-egg problem to solve there before anything like this can work