Source: Compete, a Kantar Media Company custom_html> One in five clicks in the U.S. happens on Facebook, and like any mass platform, most brands maintain a presence there and carefully cultivate their fan base. But do they need to advertise as well? That's the question facing brands -- as well as investors --as Facebook prepares for its anticipated $100 billion IPO on Friday. The Facebook ideal goes something like this: a brand builds a presence there and cultivates a fan base. That fan base spreads the word about the product. When brands want more impact, or to reach non-fans, they advertise. But the reality is that much of what brands spend on Facebook isn't going into its coffers. A Facebook presence is…