Source: Shutterstock Recently, Facebook announced that it had surpassed the 300 million user mark. According to Experian HitWise, Facebook accounted for 58.59 percent of all U.S. visits among a custom category of 155 social networking Web sites in September 2009. This is an interesting stat and I would love for Experian HitWise to send the full list over, so that I can also analyze the playing field for new, emerging, and declining players across the board. The report noted that…