Users can enable the new feature, which is still experimental, from the Facebook app's settings. However, they first need to install a separate application from Google Play called Orbot that functions as a proxy for routing traffic through Tor.
Because of how the anonymity network is designed, Facebook will not be able to push notifications back to its mobile application, so users who enable this feature will need to periodically open it themselves and check for updates manually.
Tor routes traffic through a series of random computers that participate in the network and which are known as relays or nodes. The network uses encryption and is built in such a way that no relay knows both the source and the final destination of a particular connection.
The destination is known only by the exit relays which send the traffic back onto the public Internet after it's been anonymized through Tor. The final destination of the traffic, for example a website, will see a Tor exit relay as the source, not the real user's device.
Tor can help in many cases, like when you don't want your ISP, or anyone else en route, to know that you are accessing a particular website or other type of service. It can also help when you don't want that service to know your real IP address.
The first scenario is really important to journalists, activists and other people who work or live in countries where Internet activity is being monitored and certain websites, like Facebook, are banned. Under such censorship regimes the mere attempt to access a blocked service can raise suspicion and place you on a watch list.
In October 2014, Facebook made its website available at the facebookcorewwwi.onion Tor address. This version of the website can only be accessed from within the Tor network. The traffic is no longer passed back onto the Internet to reach the facebook.com website, so Tor exit relays are removed from the process.
For now, when Tor is enabled in the Facebook mobile app, the traffic will still eventually go to Facebook's public servers on the Internet after first passing through Tor. However, the company is working so that in the future the app will connect directly with its .onion Tor service.
It might take a few days until the Tor option will appear in everyone's Facebook app, as the company will be rolling out the feature during the course of this week.