If you've got feature parity with your regular site and all those features work as well on my particular phone as they would if I was on the regular site, AND there's a good reason for the site to need to be changed to fit a small screen with a touch interface, then, fine, show me the mobile version.
Now, for certain sites where there's a benefit, mobile tweaks can be fantastic. But even among the three you list, I've had annoyances. I wish I could remember what it was I was looking for, but I was on google.com on my iPhone the other day looking for something (an app/service from Google itself, I think), and didn't find it in the spot I was used to. I was stumped for a few moments before realizing that I was on the mobile site. Likewise, I've gone to the Facebook mobile site before, trying to do something that wasn't possible from the Facebook iPhone app, and been confronted with the mobile site, which also didn't have the feature I was looking for.
So I guess my feeling is this: while I was initially excited about the idea of mobile-specific sites, I now feel similarly about them as I do about Flash heavy sites: in theory, if everything is executed perfectly, they're fine. But perfection is very rarely achieved, and so it ends up screwing the user up in some unexpected, user-unfriendly way right when you need it not to. So my initial inclination is very much against more "mobile sites" (hey, if it really needs a touch specific interface, an app from the App Store is preferable -- yes, I know I'm being inappropriately iPhone-specific here!), though in favor of small mobile-specific tweaks to normal sites. Changes in font size or moving one column/sidebar somewhere else? Slight interaction changes to make things more appropriate for a tap interface? Sounds great! [And it strikes me that putting these at the level of CMS templates or design frameworks and the like might be the best route.] But a totally different site? Almost never necessary nor even an improvement when it's being used on a sufficiently sophisticated phone (which, they all will be, very very soon).
I know much of this overlaps with what you're talking about and forgive me if I'm being too kneejerk just to the idea of "more mobile-specific sites". It just always stings doubly bad when somebody has poured a ton of work into something that tries to make your life easier but in practice actually makes it harder.
Thanks!
