I'm going to jump into the mix, but my post is short and sweet. In fact, I wrote it while waiting for tests to run.
Facebook never bet on HTML5. Anyone remember the convoluted rambling of Dave Fetterman at f8 developer conference last year? I remember being both dumbfounded and confounded by it.
HTML5 is probably the way that we should have done it.
Near as I can tell, Facebook invented their own PhoneGap (i.e., FaceWeb) and sent m.facebook.com to it, then FaceWeb changed webkit's rendering logic* to match m.facebook.com instead of changing m.facebook.com to a mobile HTML5 site? That's not HTML5 - that's just stupid.
* UPDATE: as pointed out in comments, this isn't exactly accurate. Faceweb does some funky stuff though, and their engineering manager in charge of the project couldn't even articulate what it is. So it still sounds like bad engineering unrelated to HTML5.
* UPDATE 2: You should read Matt Asay's analysis of Facebook's HTML5 fiasco. In fact, you should read everything Matt Asay writes.