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.