At first, I thought the best way to introduce myself to the topic was to learn the current version of the standard authentication protocol OAuth 2.0; but I was wrong: authorization and publication system of these social networks are quite different from each other. And although ultimately all of them use some version of OAuth, it's not worth studying the entire protocol; it has many use cases, is complex.
In short, opt for studying these services independently and sequentially. As each has enough to tell, I created an entry for the system:
Links about OAuth
Official site: OAuth 2.0 — OAuth