If you or your business idea wish to succeed and attain this goal or directive you desire, then the simplest solution would be to make this social media sign-in idea more appealing or rewarding in any way possible for the clientele you seek.
What is the point of anyone doing anything out of their usual safe known practice when it heightens the risk factor in any way whatsoever. After all, this is the World Wide Web.
Security is a major issue with social media, Facebook especially. As you can see pointed out clearly by the responses to your question "idea", if there is no reward and no apparent benefit then it is highly likely that no social media sign in will occur.