They managed to recruit semi-famous computer scientists like Simon Marlow and Bryan O'Sullivan (well, buy his company and he stayed), both of whom could get jobs at basically any company.

This doesn't mean that the average engineer is any good there, just that they do have exceptional ones too.

You'll find a lot of very insightful blog posts on HN written by talented engineers that work at FB or are ex FB..

It's quite common knowledge that a lot of brilliant engineers are hired by FB. I am by no means a fan of FB but it's a known fact that they hire many excellent engineers.

They don't need to hire Dennis Ritchies, they need to hire people who are good enough to achieve their business objectives. Facebook's apps work, their site is performant, they are not drowning in storage costs, etc... they have the engineers they need. Their business problems are not due to engineering.