More to the point, those companies use lots of other software products, memcache among them, to improve the scalability of MySQL. It could be possible, if the numbers in the article are an indicator, that these companies are spending a lot more money on hardware than they would if they were using a more scalable database like PostgreSQL. They also may spend less money on engineering time if they didn't have to configure and manage all the software needed to get MySQL to scale.
More to the point, those companies use lots of other software products, memcache among them, to improve the scalability of MySQL. It could be possible, if the numbers in the article are an indicator, that these companies are spending a lot more money on hardware than they would if they were using a more scalable database like PostgreSQL. They also may spend less money on engineering time if they didn't have to configure and manage all the software needed to get MySQL to scale.