The social network that emerged as the most popular in the late 2000s is Facebook
Founded in February 2004, Facebook started as a service for college students but then opened its doors to anyone to join.
By October 2013, Facebook reported having nearly 1.2 billion monthly active users. That compared to 800 million in December 2011.
The median age of a Facebook user also increased from 26 in May 2008 to 33 in October 2009, according to a Pew Internet and American Life survey.
By 2011, more than 42 percent of the U.S. population were Facebook users, according to aneMarketer survey.
By August-September 2013, 64 percent of U.S. adults were Facebook users, according to a Pew Research Center survey.
In May 2008, Facebook launched Facebook Connect, which lets other websites utliize Facebook users' profiles and networking features. A news website can have users register at the site using their Facebook accounts and then explore content on the site, comment on it or share links to it with their friends on the Facebook network.
Thus a news organization can integrate a social network into its website without having to create one itself and take advantage of the huge audience of an existing social network like Facebook.
See, for example, The Huffington Post's Social News page at which people can login using their Facebook or other social media accounts. Huffington Post credits its use of Facebook with driving a significant part of the traffic to its site.
People who use social networks like being able to sign in to websites using their social network accounts like Facebook, according to one study, while they really dislike being forced to register using the site's own signup process.
Having people use their Facebook profiles to register and then requiring such registration to post comments on stories may also cut down on the number of inapproprite comments people post. See the Poynter story about news organizations that have seen higher quality discussion by readers after switching to Facebook's commenting system. And this Poynter article about more news outlets using Facebook Connect to help with comments.
If a person's comments are traceable to their Facebook identity they may be more hesitant to make offensive remarks. And a very small percentage of people on Facebook use fake names, according to a study by Entrustet.
But also check this study by Disqus that concluded people with pseudonyms made higher quality comments than those using their Facebook identities.
For another implementation of Facebook Connect at a news site, see the News Mixer project developed by students at Northwestern University's Medill School of Journalism.
Also check out NewsCloud, an application media organizations like the Boston Globe and Baristanetare using to create community sites inside Facebook.