Facebook does a pretty good job of authentication, at least for nontechnical users, who are arguably the majority. If you sign up using VPN services or Tor, you get nagged for a mobile number for text verification. So Facebook either knows your IP address or mobile number. I don't consider that "de facto anonymity".

Now if you're skilled, and willing to invest some time and money, you can circumvent that. Or you can buy accounts, if you know where to shop. But that's not the case, for most Facebook users.