The cynic in me believes that both this and the official onion sites are baits to lure in those who typically care about privacy and thus don't want to use Facebook. By doing this Facebook allows them to gradually extend their medium of communication to FB, and thus gather more information on them.
People who care about privacy enough to stay away from Facebook in the past are going to know enough about privacy, I hope, that they'll continue to stay away in the future.
If this truly is Facebook's intention, I somehow doubt it'll be very successful. I'd like to think it'll encourage more people to start encrypting and signing their emails, although somehow I doubt that as well.
