Mastodon, using the same underlying ActivityPub has several "cross posting" solutions mostly to- and fro twitter. As does pixelfed (the instagram for the fediverse) to and from instagram.
I can only imagine that there will be some way to cross-post to facebook, meetup or EventBrite, somewhere down the line. Even if done by external parties. Especially facebook because a lot of your audience may be there already. And to many organizers "getting enough people to attend" is probably more important than being fully decentralised and privacy friendly.
The tool can be named Facebook-dl :)