It is encrypted, but when you're going to facebookcorewwwi.onion you only know that you're connecting to server that knows the corresponding private key, of which "facebookcorewwwi" is a hash. Which is already good. However, seeing EV certificate on this domain also shows you that the corresponding private key for HTTPS connection is signed by a certificate authority: that is, if you trust this CA, you can pretty much trust that you're connecting to Facebook, Inc.

To add on that explenation, EV is about identity. Its what should differential someone pretending to be facebook using facebookcorewwwl.com and the real entity. Its less about encryption and keys, and more about a CA claiming to have looked at the companies registration papers and other government official documents and thus confirmed their identity.

Probably authentication of server, but client still remains anonymous.

That's true. Exit nodes do not play a role in hidden services. But, Rendezvous point does. So, If your node somehow selected as rendezvous point for meeting, then you could possibly sniff the traffic.

This is incorrect, exit nodes are not involved with hidden services.

Edit: the original comment I replied to stated that exit nodes could sniff traffic.

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The connection is encrypted to the service's public key, what are you talking about? Stop spreading nonsense and go read documentation.

They cant. That is the whole concept behind the word "onion" that they cant.