This is a really good idea. It will help authenticity, but not always because according to CNET.news, there are around 83.09 fake accounts in facebook. People can make fake facebook accounts very easily (we all have seen this). Steemit used to do this in the past, but now they started verifying with the email and phone number both. In facebook you can create a facebook account just with the email address, but not in Steemit. I would say current Steemit account creation is way stronger than Facebook. If we allow people to create steemit accounts with authenticity than we will have way too many fake accounts. I know your idea is not based on the account creation service, but about to authenticate the account with third-party social media as well. We are unique and different than today's current so-called big social media platform, so I believe making ourselves better than using them to authenticate our users with their accounts. Like you did above, people can still authenticate their account.

I want to add this to your idea as well: Steemit is currently allowing third-party service to create the steemit faster with some $$ where they don't need anything, no phone number, no email. You can just request the user and master password which you can change after receiving it. That's where hundreds of accounts are being created just by giving a username. Did you know that? Facebook doesn't use steemit to authenticate their users, why do we? I like using facebook as well and want to connect with people there too. Steemit is the only platform which I'm using more than anything. I never used facebook a lot before too. It's just to connect with some people. I like them too since I can use many features which Steemit currently lacks.

Facebook estimates 8.7 percent, or 83.09 million accounts. That's a huge jump, both in raw numbers 
and as a percentage, from Facebook's last estimate. Back in March, Facebook said 5 to 6 percent 
of accounts are false or duplicate.