It's not really so much invasion of privacy if you don't accept an incoming video call. If you don't know who it is that is sending the video call, don't accept it, just delete and/or block.
I don't know the details about this new feature, but I would guess the video call feature would be limted only to between people who are already listed as friends on FB. It's like we have the chat thing but non-friends can't just hit us up to chat.