If there were any signs to suggest that Facebook is building a monopoly over U.S. social networking, the latest statistics published by Hitwise surely paint a grim picture for Facebook’s competitors. Facebook increased its U.S. market share from 55.15 percent in August to 58.59 percent in September. Where did the majority of that share come from? Myspace, not surprisingly.

It’s also becoming clear that Facebook is expanding it’s demographic base to those that are over 55, up 108 percent in the past year. While MySpace has more time spent on the site than Facebook, Facebook will most likely surpass MySpace in overall time spent each month at some point over the next few months. Also somewhat surprising is the amount of traffic to social networking site, Tagged.com.

Tagged is known for blasting out large volumes of spam to users, which is why we’ve mostly dismissed the company for not having a sustainable future. While the company is profitable according to sources, we aren’t quite sure that sending out tons of email spam is an effective way for growing a company. Then again, they may be taking advice from Facebook who has used their contact import tool as their primary source of growth.

Also not completely surprising was more data suggesting that Twitter lost market share in September. While it’s difficult to measure Twitter activity simply based on traffic to the company’s website, one has to wonder how the company will make money if people stop visiting the site. For now Twitter has a lot of money in the bank and plenty of buzz that it’s unlikely the company will fade away.

It will be interesting to see how these trends continue over the coming months as Facebook races toward 400 million users.

The Facebook vs Twitter Image was created by JESS3