The reason this is the most important takeaway is that Scalzi is calling the people at Facebook "stupid", despite the fact that they are much smarter than him.
"there’s very little Facebook does, either as
a technological platform or as a company, that doesn’t
remind me that “banal mediocrity” is apparently the
highest accolade one can aspire to at that particular organization."
Scalzi has never heard of Hiphop, Thrift, or Haystack. He hasn't heard about them because he is not really capable of understanding the technical problems involved with doing something as hard as Facebook.
It is really hard to make something look this easy.
First, he wasn't calling the people who work at Facebook stupid. Second, I have no idea how smart John Scalzi is... and you probably don't either.
He was criticizing the idea of Facebook. Facebook is a very well implemented system. But that doesn't mean that you can't question the validity of the system. Just because it's hard to build doesn't mean that it is worthwhile to use.
Also the fact that Facebook have made accessing vast amounts of information, organising events, and communicating far more simple and intuitive than any system previously available. Much as there are things to improve - there is no place on the internet better suited as a portal to your life.
It is really hard to make something look this easy.