"I knew of some Chinese migrants to Australia who watched a Tiananmen 10-year anniversary documentary, and apparently tears just streamed down their faces.
They had no clue that it ever happened."
I mean just about the specific case, I know that the Tiananmen square incident is unknown to almost all Chinese.
Even those whom I suspect may be aware of it seem to confine it to a cultural blindspot approaching wilful ignorance. It is something you simply _do not talk about_ in China.
It is pretty easy to find Chinese people talking about how few people (specially younger people) know about it too:
http://www.quora.com/China/What-percentage-of-Chinese-know-a...
http://observers.france24.com/content/20080605-tiananmen-squ...
Even those who have heard about it though, many probably haven't seen the pictures to truly understand the scale of it.
A more apt US analogy might be how many people were aware of the scale of protests against the Iraq war -- especially the vast opposition to it in Europe. My impression is that US mass media self-censored and barely reported those protests. (I may be mistaken: I don't live in the US, but that's what I've heard from people who do.)
Of course it's a far cry from self-censorship by the mass media to state censorship of the Internet. But for many people it results in the same distorted view of the world.