Tor means they can sneak through state-level internet blocks and surveillance. Once they're on Facebook, they're (ostensibly) obeying their Real Name policy, so Facebook knows who they are. Their Tor support doesn't protect their identity or communications from Facebook itself, other than obscuring their IP address.

Encrypted chats, on the other hand, treats Facebook itself as untrusted. It lets people use Facebook without Facebook knowing exactly what they're up to.