Use username provided by other social sign-on systems?

Something I thought of while filing gitlab-ee#566: Not everyone knows their LinkedIn username, because, IIRC, it has to be manually set. Maybe it's forced now? I don't know. Maybe it can be extracted from the social sign-in stuff instead of being user-provided? Same sentiment applies for the other social sign-in things. It'd be nice to sign into those and then be able to choose what I want to show on my profile instead of being limited to what I fill in. Maybe a "populate from social sign-on" function? Having the ability to fill-in might still be nice for non-personal accounts.