Soundcloud -> @funkwhale
MeetUp -> @GetTogetherComm
LinkedIn -> @cloutstream
Google Calendar -> @calendar_social
Anything missing from the list? What else is needed?
Friendica ( @news ) is maybe a better bet, it is a Facebook-style social network due to get ActivityPub at some point in the future (though at the moment they're focusing on a big database update). Also, Friendica is already compatible with Mastodon through another protocol.
Diaspora sound like they don't want to use AP at all?
One of the first Mastodon instances, there is no specific topic we're into, just enjoy your time!