Do you remember social media before the "#" sign? The pound sign, the hashtag, the bit of punctuation that enabled the widest mass searches with the absolute least technical or search background?
If you do, it was prior to 2007, when Chris Messina stumbled onto the practice, suggesting people use it to "categorize" content and make it easy for people to find that for which they were looking. It took, and the organic growth has literally changed the way we broadcast, sort, and search for content. Not just on Twitter, but across virtually all relevant social media platforms, as well as traditional broadcasting.
From a marketing perspective, it's a really interesting story. And one that leads to really fun things like following today's hashtag, #letslaughday. Go, click, have fun!