Assuming you mean an actual blockchain and aren't just using it, as some people seem to do, as a catch-all term for cryptographic signatures, it's expensive overkill. Social media really doesn't need to pay the brutal computational costs to solve the Byzantine generals problem. You don't need to establish an immutable record under adversarial conditions because there's no "double spend" problem. A simple synchronization mechanism for cryptographically signed posts (to prove authorship) is enough.

Because blockchains are definitely not built for that..?