Hindsight is 20/20. You can look back on many innovations and say that they'd be impossible to imagine in the past, but that doesn't have any bearing on the inherent viability of a given idea. I'm not saying that the Zuck/Chan moonshot isn't a worthwhile endeavor, only that examples of past skepticism prove nothing about what is actually possible.
There are lots of extraordinary claims being made everyday. It's the correct rule of thumb to be as skeptical possible, for there are many more outrageous claims than there are legitimate ones.