Facebook has launched its own hidden service on the Tor network. The service provides HTTPS access using a CA signed certificate, is not the norm for such services. This blog post is a good backgrounder and explains pros & cons of Facebook’s security/authentication choices.
Called Personal Onion Router To Assure Liberty (PORTAL), the project is a pre-built software image for an inexpensive pocket-sized “travel router” to automatically protect its owner’s Internet traffic. Portal provides always-on Tor routing, as well as “pluggable” transports for Tor that can hide the service’s traffic signature from some deep packet inspection systems.