One thing I was wrong about over the last few years: sign up costs would dramatically reduce spam. 

Turns out spam is a top 3 problem (aside from retention and infrastructure scaling) to solve for when building a permissionless decentralized social networking protocol.

Spammers are willing to pay for sign ups at prices that normal users aren't.

Spam is also relative: what's spammy for one person is not for another.

Corollary: when you talk to developers building on Farcaster, spam is a top of mind issue whereas users giving product feedback but not actively building in the ecosystem tend to think this isn't that big an issue / not that hard a problem to solve. 

Also a good proxy for the quality of first principles thinking when suggesting "why don't you just do this?" if you haven't considered how would spammers abuse this and what's the solution.

I’d put an escrow behind quality users, and risk losing it if proven wrong.