Ever since the early days of Twitter hashtag use, major events had organic hashtag communities form around them (essentially back channel discussions). Over the years, as the use of Twitter grew among brands and organizations, the use of top-down branded hashtags became increasingly common. Fortunately, thanks to the confusion and audience dilution this led to (e.g. NBColympics, ABColympics, CocaColaOlympics, etc…) most major events still see a community grown hashtag emerge as the victor for the main discussion. Of course this now applies to nearly all industry related hashtags unless of course a particular organization or brand pioneered a specific topic.
For those of you that still find it hard to determine which hashtag to use to reach an audience following a specific event, I have created a short video tutorial demonstrating a free tool called Hashtagify.me , which allows you to do this. Enjoy.