One way you could trace this kind of thing is to do a search of something like YouTube comments and look at the dates when a phrase starts to be used. It doesn't really work with 'so', obviously, because it's one word that can be used in other contexts to the one you want, and also I don't think people really write anecdotes in YouTube comments, but it would work with a phrase or a new word.

I did this with a word that I remembered was a very popular slang word through my teenage years that changed meaning several times in the space of about 4 years. It went through a phase of being derogatory, then it was used only in a reclaimed ironic way, and then it changed again slightly, then it went out of use. I wondered whether it had been just my school where this word was widely used and whether the same changes in meaning had taken place everywhere (in England, anyway) and from doing Google searches of YouTube comments I found that it wasn't just my school and that the meanings were the same. I think things like YouTube and MySpace, which were the main sites that came up when doing a general Google search, were the main reason it could spread and change meaning so quickly.

Comment 1 by Michael Austin :

I'm America, many teenagers say "He was like" instead of "He said." This seems to be pretty new, I've only noticed it in the last 5 or so years.

It's not just America! I don't mind this at all but what I do mind is sentences with 'like' every other word as punctuation.