This was the original premise of app.net - a social service from years back. They built a “social backbone”. They offered you a single place where your identity and friends were housed. Other people could build apps on top of the backbone.

So you would join say a clone of Instagram and all your friends were still there. And your account still worked. Or they had a Twitter clone. Same deal. It was a single sign-on social account/identity/social graph that was separate from the apps. So things could just plug in.

Worked great. But it was a paid service. And came out right at peak Facebook so it died off.