Coincidently I work on performance monitoring at Facebook. We gave up logging perf data to MySQL because it was a management headache at scale. So instead my colleagues built Scuba, which is essentially a search engine they taught to perform aggregate functions.
Lots of people gave up trying to do their own performance logging, it turns out its a non trivial problem when you get to large scale systems. Which is why the company I work for exists.
I am in no way an official spokesperson for them, just a developer, I don't know how much publicity I am allowed to draw to our back end architecture without getting into trouble. It is after all the thing that separates us from everyone else out there. (BTW I have posted the Who's Hiring post a couple times so my Username is pretty easy to track to whom I work for)