X is still leading at least for companies. Problem with Nostr is that there is no algo and it's not easy to find content. Ppl have to know you. 
What ppl also do not get is that decentralization doesn't mean privacy. Nostr is a good example that it can harm your privacy a lot.
Mastadon is still used a lot by privacy guys and hey, it is also possible to host it by yourself etc. 

 Great take from you!