Pretty much everyone has JavaScript. While for what you are doing, probably everyone in your target audience has Flash, but keep in mind for different versions the % of the general population that has it installed is not 100%: http://www.adobe.com/products/player_census/flashplayer/vers...

Adobe (who has an incentive to inflate their numbers though I'm not saying they are) puts Flash 9 at 84.0%. Below that you get into the high 90s. But if your whole product is based on a viral loop, and you can achieve the same thing with JavaScript, that few % can be important when compounded over time.