FireChat by Open Garden is an app that lets people communicate with each other without needing a cell network.
It is able to do that because it can make use of Bluetooth, which has a range of about 200 feet of anyone else using the map. It also works with phone networks and wi-fi.
There are versions for Android, Apple and Windows smartphones and tablets.
According to the Taipei Times, over 100,000 people in Hong Kong downloaded the app in 24 hours, last Sunday. One person said people are downloading the app because they were worried the authorities might shut down the networks.
FireChat is apparently popular in India because of poor connectivity.
Some analysts are speculating that the company could be the subject of acquisition because big players like Google and Facebook have the ability to scale such apps globally.