I still agree that they are variants of the same fundamental problem (a single credential protects all of your logins) and that Password Managers are a vastly superior solution to this problem.
But it is worth pointing out that for the layman, using Sign in With Facebook/Apple/Google, is better than single credential re-use.
When I say "layman", I mean people like my mom and grandma. I have tried to get my mom to use a password manager (went as far as to set it up for her, and pay for it) but she just reverts to a simpler solution (which is Social SSO). If she weren't using Social SSO, she would be using her same Facebook password for every site on the internet. So as much as I personally loathe Facebook, I do trust Facebook for securing my Mom's credentials far more than the random scrapbooking website she is creating an account for. In this case, I am grateful that she is using Sign In With Facebook, even though I would never consider such an action for myself. So it is a small step in the right direction.
Granted, their only job is to secure your passwords, but it's effectively equivalent to a single SSO service from a protection standpoint (if all your accounts would accept that SSO login).