In the Netherlands SSNs are not unique, they handed out some duplicate ones back in the day. So not great as a primary key. Besides, I think using them as primary keys is illegal anyway.
In Denmark they are specifically not allowed to be used as a primary key. I mean many do, but technically your suppose to have a separate internal ID and then you use the SSN to look up that ID.
SSN's predate sign-in and the internet. Anyway there is a digital government sigle-sign-on solution called DigID, but you need to create an account etc for it using an e-mail address as the sign-in.