So, it turns out that the good old Facebook 'Like' buttons and Google's 'Plus one' buttons don't behave well on HTML5 format websites. That's because they make use of XHTML and namespaces. Sure, mixing XHTML and HTML5 together on a single page works just fine for the end-user, but when it comes to validating your pages on the W3C, it just won't work.
Wrap your button embed HTML in a
Becomes this code:
Of course, this being a hack, I can't really speak for this being good practice, but it works. Evaluating the pitfalls will come later, I'm sure!