Okay so they have wide authentication infrastructure, that much is obvious. However consider this:
If users, say, want to use an alternative application to view their Instagram posts and Facebook posts, or an alternative application to interact with TikTok, you would, of course, need to authenticate the user against their service to read the APIs, and thats fine, applications can do that with user consent, and of course, if you do that, you can reverse engineer the platforms formats, as described in this article.
The dilemma isn't being able to authenticate a user against an API, its that, if you actually go through and do this, Facebook, TikTok, Slack, or any other service, can use the courts to shut down your business. They have, effectively made this illegal, or at best, legally dubious, through case law and legislation. This was not always the case however, it hasn't always been this way, see Apple reverse engineering Microsoft Office document formats, or Airbnb scraping Craigslist to build their listings.