Blockchains are immutable, which means once data is recorded, it can’t be removed. The idea that blockchains will be used to store user-generated data for services like social networks has enormous implications for user safety. If someone uses these platforms to harass and abuse others, such as by doxing, posting revenge pornography, uploading child sexual abuse material, or doing any number of other very serious things that platforms normally try to thwart with content-moderation teams, the protections that can be offered to users are extremely limited. The same goes for users who plagiarize artwork, spam, or share sensitive material like trade secrets. Even a user who themself posts something and then later decides they’d rather not have it online is stuck with it remaining on-chain indefinitely.
Nothing is forgotten on the blockchain
Once something is recorded in the blockchain ledger, it is almost impossible to remove (except, say, for a community-agreed-upon hard fork of the ledger). All of the ills of social media become even more permanent when recorded directly in a blockchain.