https://gdpr-info.eu/art-20-gdpr/

In particular, there is this wording:

"In exercising his or her right to data portability pursuant to paragraph 1, the data subject shall have the right to have the personal data transmitted directly from one controller to another, where technically feasible."

This could, in principle, require Facebook to automatically broadcast your posts to your friends on third party social networks, once a standardised technology for that becomes widely implemented. Fortunately it seems that progress is being made on that:

https://activitypub.rocks/