I'm somewhat bullish on the promise of permissioned blockchain systems, but the Facebook/MySQL example is misleading --

* Facebook doesn't need MySQL to interop with other systems in the same way that blockchain enables.

* Relational semantics are way more complex than what most distributed ledgers technologies offer. A blockchain is more analogous to the MySQL binlog than to MySQL itself. I would guess that Facebook's DB transaction log is orders of magnitude larger than all the blockchains in the world combined.

* Facebook's challenges with MySQL are most likely due to their unprecedented scale. If one were to layer on the governance challenges associated with distributed ledger systems, I would think an upgrade like that would basically be impossible.