The reason Twitter, not Facebook, will “define” social media, is because it has already figured out a way to monetize mobile, and because it is one of the most important social networks on the web, Fortune argues. And it doesn’t have a chunk of its revenue coming from Zynga. So the argument goes if Twitter manages to go public successfully, that could help validate social media as a business after they’ve been battered in the post-Facebook IPO world. [Source: Fortune]