Proponents of social networking sites say that the online communities promote increased interaction with friends and family; offer teachers, librarians, and students valuable access to educational support and materials; facilitate social and political change; and disseminate useful information rapidly.
Opponents of social networking say that the sites prevent face-to-face communication; waste time on frivolous activity; alter children’s brains and behavior making them more prone to ADHD; expose users to predators like pedophiles and burglars; and spread false and potentially dangerous information.
SixDegrees.com, which existed from 1997-2001, is considered the first social networking site because it allowed users to create personal spaces and connect to friends online. Friendster, created in 2002, popularized social networking in the United States but was quickly outpaced by other social networking sites like: MySpace (2003), Facebook (2004), Twitter (2006), Pinterest (2009), and Google+ (2012). Facebook reported one billion monthly users worldwide on October 4, 2012, making it the most popular social networking site with one in seven people on the planet using the site.  Every day, Facebook manages 2.7 billion "Likes,” 300 million photo uploads, and 2.5 billion status updates and check-ins.  Twitter, the second largest social networking site, had an estimated 107.7 million users in the United States (as of Jan. 31, 2012)  and 500 million worldwide users (as of Sep. 28, 2012).  Pinterest is the third largest social network with 23 million unique visitors in July 2012, followed by LinkedIn, Tagged, Google+, and MySpace   59% of all Internet users use at least one social networking site and 56% of social networking users are female. 
As the sites have become increasingly popular, the user base has expanded from teenagers and young adults to include more people over the age of 50. Although Facebook began in 2004 as a site for college students with log-ins restricted to those with .edu e-mail addresses, as of June 16, 2011 33% of users were 23-35 years old, 25% were 36-49 years old, 25% were over 50, and only 16% were the traditional college-aged 18-22 years old.  According to a Dec. 2012 Nielsen "Social Media Report," 20% of time spent on a home computer is on social media while 30% of mobile Internet time is spent on social networking sites. Total time spent on social media via mobile and home devices totalled 121 billion minutes in July 2012 (compared to 88 billion minutes in July 2011). In July 2012 Americans spent 74.0 billion minutes on social media via a home computer, 40.8 billion minutes via apps, and 5.7 billion minutes via mobile web browser for a total of 121.1 billion minutes on social networking sites. In July 2011 Americans spent 59.5 billion minutes via home computer, 23.2 billion minutes via apps, and 4.4 billion minutes via mobile browsers for a total of 88.4 billion minutes. 
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Facebook profile of Mark Zuckerberg, Founder and CEO of Facebook. Source: www.facebook.com/zuck (accessed Nov. 9, 2012)
Social media's largest source of revenue is advertising. Twitter, YouTube, and Facebook profit from advertising while LinkedIn profits from ads, subscribers, and selling data to third parties. Social media ad revenue is expected to reach $8.8 billion in 2012, an increase of 43% from 2011.  Gaming also accounts for a large portion of social networking revenue, with an expected $6.2 billion in 2012. Predictions place overall social media revenue at about $34 billion by 2016, an...