Of course it's hard for me to compare because HarperCollins has launched their effort as a "by invitation-only" site. They are currently accepting e-mail addresses but won't guarantee when and if you get an actual invite. That might be just because the site is still in beta mode, or it might be an effort to make it more like a "network" than an open-to-all kind of place like OWW or Critters. Either way, it's interesting. I'll keep an eye out (and ask MHF to do the same) for more on this site.
Personally, I blame (or credit) this (and maybe ABNA) on the success of American Idol.