Nostr is the number 2 decentralised social media network after Bluesky according to this chatGPT research:

1) Nostr overall (protocol, not just Primal)

• Nostr itself — the decentralized protocol underlying apps like Primal — has been reported with millions of users across clients. One source cites over ~18 million registered users on the Nostr network as of mid-2023, across all clients and relays. 

2) Mastodon (Fediverse, decentralized)

• While not “Web3” in the blockchain sense, Mastodon is a decentralized social network and has historically seen millions of users. It surpassed 10 million registered user accounts in 2023 and had peak activity metrics in the millions. 

3) Bluesky (AT Protocol)

• Bluesky isn’t blockchain-backed but is decentralized by protocol (AT Protocol). It has tens of millions of users — e.g., around ~40 million by late 2025 according to recent reporting.