The growth of social networks significantly contributed to the effectiveness of viral marketing. As of 2009, two thirds of the world's Internet population visits a social networking service or blog site at least every week. Facebook alone has over 1 billion active users. In 2009, time spent visiting social media sites began to exceed time spent emailing. A 2010 study found that 52% of people who view news online forward it on through social networks, email, or posts.
Publishers can offer advertisers the ability to reach customizable and narrow market segments for targeted advertising. Online advertising may use geo-targeting to display relevant advertisements to the user's geography. Advertisers can customize each individual ad to a particular user based on the user's previous preferences. Advertisers can also track whether a visitor has already seen a particular ad in order to reduce unwanted repetitious exposures and provide adequate time gaps between exposures.