Exactly, if you want to create a website for your high school chess club there is a ready made solution that while not perfect is fairly difficult to beat by hand. So the average became more complex because the simple problems had already been solved AND you are building generic solutions to cover more areas. In other words Facebook solved a lot of the problems people used to learn HTML to solve and now Facebook needs a few high quality designers not just a HS student with some time on their hands.
Net result fewer people learned HTML because it was not needed.
You also have Microsoft making efforts to obscure HTML completely to create lock-in to Microsoft's design tools. It's long been easy to create a "web page" with Frontpage or Word as a WYSIWYG editor. The followup was ASP.NET Web Forms, where a designer uses the graphical toolbox to make all the interactive fields and buttons and such without knowing a lick of the underlying HTML. The net effect is to reduce the amount of HTML knowledge in the marketplace.
As someone who currently works in Microsoft's web ecosystem, I would have to disagree. Microsoft's tools encourage you to know the underlying HTML and "how things work" under the hood.
I'm pretty sure it's going to be a tough road to slap together a site with Visual Web developer or Visual Studio without knowing the "guts" of HTML/CSS/JavaScript, C# or VB, an ORM, and and a smattering of SQL.
Heck, even their Expression Blend and Visual Studio WPF and Silverlight designers assume you'll want to edit XAML by hand. Microsoft's current generation of development tools emphatically are not in the business of hiding markup language away from the developer.
Net result fewer people learned HTML because it was not needed.