There's technically nothing to prevent sending messages from facebook to twitter etc, those companies just didn't want it to happen so they didn't agree on a way to do it, that is a protocol.
Whether Facebook and Twitter end up supporting such a standard may well depend on how expansively the European Court of Justice interprets the right to Data Portability that is being introduced by the GDPR:
https://gdpr-info.eu/art-20-gdpr/
Certainly when read in the light of existing competition law (not to mention the bad reputation Facebook in particular has right now, nor any cynical political protectionist goals that an EU court might have), it is possible that large near-monopolistic American social media companies may end up being forced to allow automated pushing of user's posts to friends on 3rd party competing websites.
But not every transaction is a valid vote.
The middle ground is instead of a few major players in a given market, we have thousands of players in a market that people can move between easily. Kind of like mastodon, but every node is a different community with different interests. It would be like if we “upgraded” all of the existing forums on the internet today.
So instead of Facebook, we should have 10,000 smaller “social networks” all competing. This sounds like a much better outcome than never trying to compete with Facebook or twitter or google.
How can these large tech companies today serve the interest of every “user”? They can’t.
Decentralization is nice, but you don’t even need it to solve this problem.