Some information on the Web:
MySQL at Facebook's page https://www.facebook.com/MySQLatFacebook?filter=1
A post by Ryan Thiessen, Database Operations at Facebook on Quora: http://www.quora.com/Facebook-Engineering/How-does-Facebook-structure-MySQL-so-that-it-is-robust-and-scalable
“A lot of people are surprised that for this shiny new thing for
Facebook, we’re using MySQL,” Piantino says. “We treat [MySQL] as a
generic engine for data manipulation. We use it as a storage engine.
And it’s really efficient.”
In 2008, Piantino saw a presentation by an engineer from InnoDB — an
outfit that does storage engines for MySQL. He remembers thinking that
if he was ever trying to solve the problem of finding data on disk,
there “wasn’t a chance” he’d come up with a better way than the engine
InnoDB had built for MySQL.