Is there a general #ActivityPub implementation that can serve as catch-all backend which can then be used by different looking frontends (twitter-like, instagram-like, etc)?
@humanetech Cool! Thanks, I'll have a look!
@dejfcold Pleroma is the closest you can get. It has swappable frontends, though most are focused on microblogging.
@sean is working on a frontend for more long form blogging that he’s said is supposed to be Diaspora-like. I’ve heard of various frontends that were Instagram-like, but haven’t seen any recently.
I was once suggested Pleroma for selfhosting. I'll look at it in more detail.
I was hoping to selfhost only one backend and use different frontends for it so I could still use it like instagram-facebook-twitter stack but not waste server resources running multiple backends. Thanks!
Server run by the main developers of the project 
It is not focused on any particular niche interest - everyone is welcome as long as you follow our code of conduct!
@dejfcold take a look at #gofed
It is designed for that purpose, where you can design your own #activitypub extension as #jsonld extension, #golang code generate, and build on top of that.
A stack of libraries will ease the work, abstract away the complexity of both #S2S and #C2S federation.
I created a diagram of this stack..
https://go-fed.org