Are Facebook page builder companies dying?

I think you partially answered your question, but I think some of it has to do w/ Facebook's growth overall. At the time of that article, I think company's offering the service preyed on an assumed barrier to entry. Over the past few years, as even more people of all ages became more accustomed to it, they felt less inclined to bring in an "expert."

Facebook is still growing and have not quite finished defining itself yet. Further, Facebook as it is, is not really a platform. So, as it grows and introduces new features to enhance user experience, the third-party developers will become obsolete, one by one.