Social media can be understood as a shift away from media oligarchy, in which average persons are almost exclusively the consumers of information, toward media democracy, in which they also become the producers, publishers, and syndicators of information."
— Mark A. Foster, Ph.D., Social Media.

Think of regular media as a one-way street where you can read a newspaper or listen to a report on television .... Social media, on the other hand, is [sic] a two-way street that gives you the ability to communicate too."
— Daniel Nations, "What is Social Media?" About.com. (Retrieved on December 25, 2009.)

Whereas much of the activity on the worldwide web to date has been of a broadcast nature (that is, one to many), social media is [sic] seen as many to many, because of the way in which it relies on the links between peers and aggregagors of content for its distribution. For example, a conversation on a single topic (or meme) can be spread across multiple Internet locations, but is joined together by links in a way that ensures no one voice becomes the authority.
— Niall Cook, Enterprise 2.0: How Social Software Will Change the Future of Work. Burlington, VT. Gover Publishing Company. 2008.

Social Media: Online technologies and practices that people use to share opinions, insights, experiences, and perspectives with each other.
— "Multiplatform Glossary." Television Bureau of Advertising Online. (Retrieved on January 7, 2010.)

Social Media is, at its most basic sense, a shift in how people discover, read, and share news and information and content. It’s a fusion of sociology and technology, tranforming monologue (one to many) into dialog (many to many.)...

1 - Social media describes the online tools that people use to share content, profiles, opinions, insights, experiences, perspectives and media itself, thus facilitating conversations and interaction online between groups of people. These tools include blogs, message boards, podcasts, micro blogs, lifestreams, bookmarks, networks, communities, wikis, and vlogs.

A few prominent examples of social media applications are Wikipedia (reference), MySpace and Facebook (social networking), Twitter and Jaikue (presence apps), YouTube (video sharing), Second Life (virtual reality), Upcoming (Events), Digg and Reddit (news aggregation), Flickr and Zooomr (photo sharing), Blogtv, Justin.tv, and Ustream (livecasting), Stickham, YourTrumanShow (episodic online video), Izimi and Pownce (media sharing), del.icio.us (bookmarking) and World of Warcraft (online gaming).

2 - Social Media is the democratization of content and the understanding of the role people play in the process of not only reading and disseminating information, but also how they share and create content for others to participate. It is the shift from a broadcast mechanism to a many-to-many model, rooted in a conversational format between authors and people.
— Brian Solis, "The Definition of Social Media. WebProNews. (Retrieved on January 7, 2010.)

Social media are online communications in which individuals shift fluidly and flexibly between the role of audience and author. To do this, they use social software that enables anyone without knowledge of coding, to post, comment on, share or mash up content and to form communities around shared interests.
— Joseph Thornley, "What is 'Social Media'?" Pro PR: Exploring Social Media and Public Relations. April 8th, 2008. (Retrieved on January 7, 2010.)

Social networking is the grouping of individuals into specific groups, like small rural communities or a neighborhood subdivision, if you will. Although social networking is possible in person, especially in the workplace, universities, and high schools, it is most popular online. This is because unlike most high schools, colleges, or workplaces, the internet is filled with millions of individuals who are looking to meet other people, to gather and share first-hand information and experiences about any number of topics ... from golfing, gardening, developing friendships and professional alliances.
— "What is Social Networking?" (Retrieved on January 7, 2010.)