1) Facebook 2) Myspace 3) Youtube 4) Wikipedia

I'd say that's about it...those are the only ones where everyone knows what they are because they are constantly talked about in the mainstream media

HotOrNot, Slide, Flickr, Digg, Reddit, Last.fm, WordPress etc.

These are ones we know because we are in a western society. There many other big hits in other countries that we have not heard of as they are not reported by western media e.g. until recently I never heard of shaadi.com and thats a big Indian player with a western bias which only gets reported as it is in English language and also doing well in English speaking countries

There are many more big players in China, Russia etc., all depends on what your assessment criteria is as they did not mention that in the article.

no these are the ones you know because you are an internet junkie. Your average person in the real world never heard of any of those. The ones I listed everyone knows because they either use them every day, or hear about them during the day.

What about the examples they used in the article?

Looking at Alexa's top 50, Flickr seems to be one of the few web 2.0 companies up there. I'm disqualifying Wordpress.com because of the mass amount of blogs hosted there, which if is considered web 2.0 then Blogger would have to be included as well.

Theres a couple ones you missed out on.

Ebay, Amazon, Cragistlist? and with the plethora of dating sites out there, theres a couple that could be considered "main stream"

Finally, as much as I'm not fond of piracy, you'd probably have to put The Pirate Bay in with that lot too.

It really depends on what you call "mainstream" and the media you consume, watch, or listen to.