Facebook's use of MySQL makes total sense just like their use of PHP does. It was written in those languages/technologies way back when and while it might all be held together by duct tape and bubblegum, a wholesale rewrite is a TERRIBLE idea.

Does Facebook run and make money? Are users happy? Is it fast?

In general if those three things are good, then why do a rewrite or a massive switch?

Facebook has very smart developers and if they though they could go out and swap out database vendors and get a big gain, I bet they would, but they don't. Why? because I bet the gain isn't as big as advertised.

VoldDB is in-memory so it's fast? Cool, but FB already uses memcached, so you aren't going to get any huge perf wins there. Maybe it would be simpler and maybe if FB started today they might use it, but the cost to swap out their proven architecture for something new and shiny probably doesn't provide a big enough user benefit for it to be worth their time.

At the end of the day, very few companies truly understand what it is like to have half a billion people logging into your website on any given day to chat, send messages, upload photos, play games, and so on. Any platform is going to have problems at that load, swapping databases doesn't solve that at all.