Yeah, a lot of things have changed since 2006. It would however been interesting to see an updated post with current versions of both MySQL and PostgreSQL.
For small use cases, e.g. Hacking on a developer workstation, MySQL has historically been a lot easier to get started with. No need for Cygwin which used to be the case for postgres, MySql has a GUI installer, mysql has a ton of online help resources, no mental drain to deal with up front - how often should I run a Postgres vacuum? How should I schedule it?
When it comes to scaling up though, Postgres has in my experience, always just worked. This has been true for years. The documentation is excellent and the community extremely helpful and knowledgable.
So that should calm any doubts about MySQL's ability to scale. Twitter and Facebook even have their own forks which are tailored to their individual use cases.
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.
Most of them use PHP, what does that say about their ability?
People choose technology that they're more familiar with, not necessary the best out there.
Let's not forget that almost all of them use some sort of caching layer in front of MySQL, let's talk about that part of "scaling" shall we?
Last but not least, forking MySQL to suit their own needs do not necessarily imply that MySQL scales at their size, which is something you cannot claim.
Closing remark: let's say MySQL does scale, what if PostgreSQL scales even better?
I'm not a specific RDBMS fans since I use whatever dictates to me (with some exception) but really, the "name-calling" of who's using what should stop.
