Basically you have e-mail, which is a protocol (actually a set of protocols, but ignore for now). Thus you can e-mail a protonmail user from your gmail/corporate account without any issues. However Twitter and Facebook are proprietary black box services and beyond using a Facebook-provided identity to create a Twitter account, there is no way to 'friend' a Twitter user on your FB account or interact with content in any way.
Then there comes a protocol called ActivityPub[1], which "e-mailifies" social networking. So far Mastodon and their goodreads clone are harvesting modest success.
I am a great fan of the goal, but not of the top-down approach W3C have taken with ActivityPub. For example LBRY solves this problem in a different way and is way more successful than all of Fediverse in total.