Not everyone who uses your app uses social media or whatever service you would link for identity. Many of the most vocal and influential people especially, tend

Not everyone who uses your app uses social media or whatever service you would link for identity. Many of the most vocal and influential people especially, tend to be somewhat eccentric or idiosyncratic, so unless you're sure of who exactly your customers are, you want to spread the widest net possible. Data control and customer privacy are probably issues as well, but if login is the first impression, you probably want it to be as inoffensive as possible.