I agree that they were probably mistaken in 'market share' when in reality it should have been market penetration. But I think they idea was to say that Facebook has data or can get data relating to 80 percent of the population, so they believe that's a good enough reason to make sure Facebook doesn't use that opportunity do to harm.

Because once they have such a high penetration rate, people would be very hesitant and afraid to leave the service if the T&C are too overreaching.

Your friend might not be poorer for it, but the chances of you (assuming that you do have an account) of rejecting a new addition to the existing Facebook T&C (and as a result quitting from Facebook) is very slim, especially since there isn't a resembling service that you might transfer to (at least not a strong service).