I hope you include the subtle parts that help make it viral. For example, a media button that posts to friends: <Happy Programmer picture> "The Khan Academy of programming, try it!"
I don't think the shape of the share button matters. What made it viral is that once you play with it for 10 seconds, you just want to share it with all your non-geek friends. Because it's fun. Because it finally lets them understand why you are having fun programming.
Make what you do fun, and it will be viral. Obsess over the language of the share button with something boring, and pull your hair for eternity.
A startup needs to understand that there's several stages to becoming the next big thing. The first and most important: Raising awareness. How can some users tell anyone if they haven't heard about it yet?
How they not only raised awareness but captured the interest of thousands is important.
