I use both. I find that sometimes one or the other will display some weird traceback, or worse, silently fail. Now my first reaction to a problem that isn't obvious is to switch debugger.
Sadly I think the world of Javascript debugging still has some way to go...
For me FB is better for prototypin code. You have the expanded console that allows me to write code quickly on pages that I may not have direct access to (like production pages). I feel like I'm using a text editor. Webkit debugger is much more limited in this regard.
IMO, yes. Maybe it's that I was on Chromium on GNU/Linux, and not Chrome proper, but I found the chrome dev tools to be buggy, and a little incomplete, like as-of-yet unpolished FB clone.
Sadly I think the world of Javascript debugging still has some way to go...