Are there only a few people who realize how big of an issue having no vender-neutral solution is? That'd actually scare me.
If you think this deeper, then why not organize the voting of the government through Facebook or Google? That'd outsource the identiy management, so it's cheaper.
It's depressing how many people are blind to consequences of technological decisions, especially when financial incentives are at play.
Most people have jobs with companies that run mailservers. There are even community mailservers. Email is a free protocol, and as such it's vender-neutral by default.
But it makes more sense in the context of a reply further down: "NYU is a private university. The social media login option is only for visitors who are engaged in onsite research. Our materials are not available to the public online." If they are a private company, of course they can do as they please.
Tbh, there are two possible explanations:
1. Someone tried to save a bunch of money and data-handling responsibility.
2. Someone sold tracking data and users to Twitter and Facebook.
Even some of the major "social" providers support this; e.g., I think Google does.