Facebook, et al, could provide restricted APIs that let another authorized service gain access to your information via your user identifier. I imagine they provide partial access for favored partners already.
What we're talking about here is pure software interop. There is no standard, and any standard that would be devised would become a quickly-outdated albatross around the neck of the developers. Imagine writing code to interface with someone else's crap API, except now you're required to do it by law, and any new feature you come up with has to be either shoehorned into it, or more likely, go through a long and bureaucratic approval process, or even more likely, simply never shipped.
Now pretend instead of Facebook, it's your instance of Mastodon that you want to hack on. HN doesn't have the requisite font face options for the magnitude of NO F'ING WAY that is my reaction to getting the government this deep into the nuts and bolts.
Cellphone carriers argued that number portability was too hard or too expensive.
"But cell phone companies argue counter that the idea of portability should be scrapped because customers aren’t demanding it. They argue that implementing portability, which involves retooling the software of each carrier’s computerized call-routing system, would be expensive and technically complicated.
“The cost to be able to move numbers from one carrier to the other is significant, and it would be significant for the customer too,” said John Scott, deputy general counsel for Verizon Wireless. “Our argument is that it’s not necessary to promote competition.”
Carriers Aim to Kill Number Portability - 2002
https://www.latimes.com/archives/la-xpm-2002-jan-16-fi-cell1...
This is materially different.