>These aren't serious competitors, and even if they were, the serious privacy advocates are all working on Nostr, so it's the only one that activists are going to use.

I'm going to argue that at this point, Nostr isn't a serious competitor to the others as far as social networks go. If security is what you're after, then sure, it's currently better in that regard, but until more people know about it and the onboarding process is easier, then that doesn't mean much. It could be the most secure platform ever built but if people find it confusing or difficult to use, or don't know anyone on it, then they aren't going to use it.