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

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/version_penetration.html 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.