That was probably because a few years back when those sites where launched MySQL was much faster. Now PostgreSQL is as fast (in many cases even faster) BUT much

That was probably because a few years back when those sites where launched MySQL was much faster. Now PostgreSQL is as fast (in many cases even faster) BUT much more robust out of the box (no corruption worries like with MySQL) and more feature rich (FTS etc is betetr in default). I think today when building, the choice is not as one sided in favour of MySQL.