I would, the very day they announced the fee, build a free Facebook clone.
While there would obviously be a market for one, what would your business model be? A free Facebook has proven to be a great way to lose money.
I don't think there is a good reason for Facebook to ignore traditional web advertising. Sure, they have the potential to do more interesting and lucrative things in the future. But, I think shunning banners now is being a little too stubborn.
I don't think that any of the new features Facebook has added over the last few years have added real value to my experience or the experience of most of the people I know. The profile, wall and photos make Facebook. I think that has always been enough. None of the other nonsense will get you laid anyway.