I'll shoot it right back at you because I don't necessarily agree with you either (all your points could be valid) but at that point, for me, it comes down to Twitter.

All the points listed are "state changes". Protestors went from a group of 10s of thousands to allegedly 3 million, Mousavi went from being a free man to being under house arrest and the elections president went from supporting the election to allegedly calling it into question.

Now, if I hear news of a "state change" such as there my first question is "what's the source. If it's an anonymous guy on Twitter I don't jump to believe it. If it's the BBC I probably think it's possibly true.

So it still comes down to cognitive process or the lack there of represented by people who believed this stuff.

Oh, and btw, since it's a pet peeve of mine. Even if some areas are 50 across I have to believe the crowd would have spilt into narrower areas (since no venue is designed to hold 3 million people). But even at a constant 50 across you're still talking about almost 12 miles worth of people.