Facebook has something like 500 million active accounts, right? If we assume that the overwhelming majority of those have a 1:1 mapping with real-world people,

Facebook has something like 500 million active accounts, right? If we assume that the overwhelming majority of those have a 1:1 mapping with real-world people, then somewhere between 15% and 25% of Internet users have a Facebook account. So, the lowest-friction way to authenticate users still cuts out 75-85% of people on the Internet. Edit: Not to mention everyone under thirteen.