To me, Wikipedia was like proto- web 2.0. That is, yes, it fits the definition, but significantly predated the trend. It was ahead of its time.

I think it's a bit fuzzy for "blogs". To me, it wasn't really until blogging became extremely accessible, that is without spinning up a website running software on your own servers, did I consider it "web 2.0". But then, yes, I agree that the explosion of blogs was fully part of that trend. (I was dismissive of this at the time, but in hindsight I think it's rad and I don't understand what my probably was.)

Definitely flickr is quintessential here!