Wikipedia
facebookcorewwwi.onion is a site that allows access to Facebook through the Tor protocol, using its .onion top-level domain. In April 2016 it had been used by over 1 million people monthly, up from 525,000 in 2015. Neither Twitter nor Google operate sites through Tor, and Facebook has been applauded for allowing such access, which makes it available in countries that actively try to block Facebook.
In October 2014, Facebook announced that users could connect to the website through a Tor hidden service using the privacy-protecting Tor browser and encrypted using SSL. Announcing the feature, Alec Muffett said "Facebook's onion address provides a way to access Facebook through Tor without losing the cryptographic protections provided by the Tor cloud. […] it provides end-to-end communication, from your browser directly into a Facebook datacentre." Its URL address – facebookcorewwwi.onion – is a backronym that stands for Facebook's Core WWW Infrastructure.
Prior to the release of an official .onion domain, accessing Facebook through Tor would sometimes lead to error messages and inability to access the website. There are numerous reasons to use the Tor-protocol for legitimate purposes, such as for increased anonymity when connecting to Facebook. ProPublica explicitly referenced the existence of Facebook's .onion site when they started their own hidden service.
Connecting to Facebook through Tor offers a way to access the site with a higher level of protection against snooping and surveillance from both commercial and state actors. The site also makes it easier for Facebook to differentiate between accounts that have been caught up in a bot-net and those that legitimately access Facebook through Tor. As of its 2014 release the site was still in early stages, with much work remaining to polish the code for Tor-access. It has been speculated that other companies will follow suit and release their own Tor-accessible sites.