....mostly because they have big enough engineering teams to roll their own and/or re-engineer an existing database to keep up with their workload.

Oracle SQL might be the fastest, most robust out of the box database, but for a huge deployment like Google/Facebook/Twitter, even out of the box Oracle SQL won't be good enough. It's kind of an unfair comparison.

That's not to say I'm defending Oracle's bullshit, but you gotta put these things in perspective.

Google and Facebook use Oracle, not for their main product but for their back-office.

eBay also used Oracle in the past (if you dig back in the past of their architecture/design of eBay).