Well MySQL scales in much the same way that SQLite does – if you build sufficient tooling around it, you can scale it as much as you like.

MySQL comes with a bunch more built-in that SQLite does, but Facebook and GitHub aren't scaling purely with the built-in functionality, they're writing their applications to be sharding-aware, they're writing custom proxies, they're running custom extensions, they're picking and choosing which bits of core functionality they allow in order to minimise scaling issues.

I think in a systems design interview these issues need to be addressed, to show that you have the skills to be able to implement this, and I think that a good interviewer should be open to this discussion!