Facebook's entire business model is collecting data on billions of people, having complete control over it, and selling it on to advertisers, marketers, political firms, ISPs, governments, and any other third party willing to pay. There is no way they are going to use a public blockchain, even for something as simple as logging in. Zuckerberg will want complete control. Either that or Zuck will want everyone to link their real identities to their bitcoin wallets so he can build even more of a profile on you.
Forgive me for being cynical, but Facebook do not and have never had the users' best interests at heart.
No need for forgiveness, Zuckerberg's not fooling anyone and you made the obvious point. Unless FB wants to change its entire business model, then there's no way it would want to starve itself of a working revenue stream. Why would they want to kill the golden goose?
Also, I'm pretty sure by now it's no longer a sole interest. FB's got so much dirt on so many people, both at home in the US and abroad, people willingly sharing information about themselves - I'm sure enforcement and intelligence also have vested interest in helping FB continue its path.
There's no altruism in any of this. The sooner we understand that, the better.