They work for us so far for users' engagement to register. 96% of registration done through social APIs.

Any idea why? Could it be a design "issue"?

That is massively out of proportion with everyone else. I suggest you look into what you broke on your normal registration process.

It really depends on the application. For a casual use app, the social logins are the smallest barrier to entry for the type of user they are targeted at.

Any "friction", like having to enter a username, email and password is three more steps than a social login requires, costing these casual users.

So, I can believe it is highly effective for specific applications.

That's a lovely theory and all, but literally nothing has a ratio of social:real registration anywhere approaching 24:1. Those casual use apps you are talking about get 70% social if they have really bad registration processes. Under 50% if they have decent registration.