Well, before bitcoin tip bots were blocked by Twitter, one could say Twitter was the first Bitcoin social network. :-)

Anyway, ... the days of signing up on someone else's website (whether that be Twitter, FB, ... Gab, Gettr, or WorldStar even whatever) are nearing the end. 

Nostr FTW!
https://stacker.news/items/6830

And then see Synonym's "Decentralized Social Media", (roadmap shows Q3/Q4 2022).
https://synonym.to/products

Zion

https://www.getzion.com
https://bitcoinmagazine.com/business/zion-social-media-on-lightning-network

Are we aware of any other Bitcoin Only Social Networks?

Best

Can you support your claim that this is the first bitcoin only social network?