http://developers.slashdot.org/story/11/07/09/1256241/Facebook-Trapped-In-My
SQL-a-Fate-Worse-Than-Death
According to database pioneer Michael Stonebraker, Facebook is operating a
huge, complex MySQL implementation equivalent to 'a fate worse than death,'
and the only way out is 'bite the bullet
Google, Facebook, Twitter if you want and others are pioneers of this era,
I think they have also money to invest in research, in fact they hire the
very talented people.
I think it is part of the game trialerror, and honestly I dont see many
alternatives to MySQL,
especially when they started.