Social media content is not a scarce, fungible resource, it doesn't require global consensus and doesn't require an arms race of useless hashing to prevent double-spends (since there's nothing to spend).

If you want to prevent others posting as you, just sign posts/actions with a private key. If you want transferrable ownership of short usernames, that can be done by having the previous owner sign a timestamped file stating the (public key of) the new owner, maybe with some hashcash thrown in to prevent a double-transfer. Since there's no need for global consensus, you can limit spam using whitelists (i.e. ignore almost everybody); it can be reduced further by adding hashcash to posts/actions.

(Yes, hashcash didn't work for email; that's because email must interoperate with existing non-hashcash senders, and hence can suffer a downgrade attack; plus email allows clients to disconnect after sending, so the servers have no way to request them to include more hashcash, which drives down the price they're willing to accept to 0).

I'm very curious how you plan to explain this to non technical folk in a way that they'll actually care enough to slough through any of that. Blech.

I don't understand; messages should be tailored to their audience, and Hacker News commenters are technical folk. I wouldn't give that explanation to non-technical folk.

My non-tech answer would be that blockchains prevent some specific types of fraud in virtual currencies (e.g. trying to create new money out of thin air). Social media doesn't involve virtual money, e.g. it's fine to create new posts, friendships, etc. out of thin air. Hence they don't need a blockchain. The problems that social networks do have (e.g. posing as someone else, sending spam) can be dealt with in other, less expensive ways.