It's not one IP! It's over 350 different IPs, all from AS32934 (Facebook).
You know how rack servers crank up the fans when under a high load? Facebook pinned the fans at max. (Sounded like a jet taking off.)
My solution: Block the user-agent string in the sites-available file and return a "418 I'm a teapot". Fans immediately dropped to the idle rate.
It's been 7 hours. Facebook is STILL trying but has slowed down to 30,000 requests in 7 hours.