the online technologies and practices that people use to share opinions, insights, experiences, and perspectives. A few prominent examples of social media applications are Wikipedia (reference), MySpace (social networking), Gather.com (social networking), YouTube (video sharing), Second Life (virtual reality), Digg (news sharing), Flickr (photo sharing) ... – Wikipedia
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, transforming monologue (one to many) into dialog (many to many). - Brian Solis
With the rapid growth of businesses using a range of social media tools, it is important that you protect your brand by ‘securing’ a presence on the popular ones. Even if you don’t intend to use it right away, at least it will be there for you when you are ready.
A great website to check the availability of your name is namechk.com.