Finally comes the 3rd party social logins like Facebook auth, Google auth etc.
3rd party social logins are almost* free but that comes with the cost of some serious databreaches and trust problems we have experienced in past, leaving small businesses and individuals no choice at all. Having access to resources over the internet is a "basic human right". But most of these services are using 3rd party authentications (since it’s free and seamless to large extent) which track, trace and misuse individuals data to earn their profit by either directly selling their data or indirectly giving it to advertising agencies without caring about privacy at all.
For an individual
Tracking & Tracing
If an individual uses social login in an e-commerce website, then the social login provider knows what colour or brand or size (I know, I am exaggerating a bit, but you got the point) of t-shirt he/she is buying.
If an individual books a flight ticket from a tour-travel website and if he had used social login, then the provider knows where this guy is traveling to and suddenly he realize that he started seeing advertisements of hotels, cab services etc related to that place. This is clearly a breach of privacy!
No control & consent
Tomorrow, if social auth provider shuts down or knowingly or unknowingly deletes the user’s data, then the user loses access to the service which he might be using for a long time. This means your access is being controlled by someone else and NOT YOU!
If you are a business and have used 3rd party authentications on your website and if these services go down then all your users will lose access to your application and hence you lose business.