what was the critical point above which the internet was officially mainstream?

The answer is only known in retrospect.

In terms of the internet, I would say the web became more mainstream when browsing services like AOL dumbed down the concept of being online to people making it a "no think, just point and click" zone.

I think systems such as CoinBase have been trying to do just that, but it's not quite there yet.

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Twitter.

Facebook is played out and everyone is dropping it because there are better places now. Plus, no one really trusts facebook. Anyone who is still using facebook does not really understand the context of technology that they are using, IMO...what I mean is that it's a psychological & experimental data gathering plan posing as a social network.

I use Twitter now...not because the privacy is all that much better (i think it is but it's arguable)...but it's a different context and usage of information unlike facebook. I also tend to find less ego and drama on twitter and more sharing of interesting ideas and info. It's not often cluttered with food and peoples personal crap (well depending on who you follow)...but I think people are inclined to use twitter in a way that happens to be easier to digest, smoother to stream, and more relevant to what people what to see. 

I tried to kick into Google plus, its just no one else really uses it it seems.