La definicion de WikiPedia dice
Social media is an umbrella term that defines the various activities that integrate technology, social interaction, and the construction of words, pictures, videos and audio. This interaction, and the manner in which information is presented, depends on the varied perspectives and "building" of shared meaning among communities, as people share their stories, and understandings.Social media can take many different forms, including Internet forums, message boards, weblogs, wikis, podcasts, pictures and video. Technologies include: blogs, picture-sharing, vlogs, wall-postings, email, instant messaging, music-sharing, crowdsourcing, and voice over IP, to name a few. Examples of social media applications are Google Groups (reference, social networking), Wikipedia (reference), MySpace (social networking), Facebook (social networking), Last.fm (personal music), YouTube (social networking and video sharing), Second Life (virtual reality), Flickr (photo sharing), Twitter (social networking and microblogging) and other microblogs are Jaiku and Pownce. Many of these social media services can be integrated via social network aggregation platforms like Mybloglog and Plaxo.
Pero si quieren ver la creatividad puesta al servicio de la explicación de este término no dejen de visitar Video: Social Media in Plain English
PD: Gracias Erica por el dato !