Shout out to AccessNow folks and @Xeniax who helped to convince Meta to unblock #deltachat invite URLs in Facebook. Previously delta invite-links on Facebook were marked as "dangerous" and didn't work.

sidenote: if you send someone an invite-link, they can click it directly after they installed Delta Chat because they will be guided through the "create-profile" flow automatically and then be dropped into a chat with the inviter.