"I don't think Mark Zuckerberg could have sat a VC down and said, this is my idea, (or this is the idea I stole from the Winkelvoss brothers or whatever) please invest in me."

Well, this is exactly what he did when he got money off Peter Thiel, isn't it?

If someone can't either explain your vision or at least mock up a tangible version of it that shows what they are trying to do, then how can they expect to create the product in reality. Sure, Facebook may not have made sense until it existed in some form, nor friendfeed, but that doesn't mean they couldn't have a good elevator pitch, the pitch is often showing off part of the product, not just an idea, which by itself is largely worthless. Most VCs don't fund just ideas, there has to be at least some basic implementation already.