Social networks need global name spaces to get mainstream adoption.

Crypto/web3. Author of Read Write Own

readwriteown.com

I don't really think this is axiomatically true. Facebook doesn't really have a notable global namespace.

1:1 (friend) networks are likely fine without, I agree that 1:many (follow) networks pretty much require it.

making data useful with AI. Previously built @stelo to keep you safe from phishing 
benscharfstein.twitter
https://nf.td/ben

Sure, the database needs to enforce uniqueness but Facebook doesn't really have usernames (at least not useful, user facing ones) in the same way follow networks like twitter, snapchat etc do.

Maybe one way of thinking about it is that when real names are used, username namespace is less important.

A global graph where any two users can be friends required global identifiers