My answer to that question is that it is a forum or channel that provides electronic peer to peer communication. A social function is a common, but in my opinion not necessary, characteristic of social media.
Thus not all e-marketing tools are social media, despite that all e-marketing offers some amount of interactivity. Electronic communication can still have the traditional marketing communications broadcasting nature: One to many. (One too many one is tempted to write.)
There is no absolute consensus on where to draw the line between social media and other e-media. But on the other hand there is no need to draw that line. You could use any channel that works for you without bothering about what kind of media it is.
A blog wouldn’t generally be considered a social media. A blog normally offers it’s readers to comment on the posts in the blog, but that doesn’t qualify the blog as a social media. It’s still mainly a unidirectional communication.
The most well known social media are Facebook and Twitter. Twitter is a service on the Internet that offers its users to post short messages (140 characters). The service was originally intended to let others know where you are at any time. Nothing prevents you from expressing your feelings or opinions instead of reporting where you are. To be able to read the messages someone posts you have to ”follow” them. That is done by a simple click in a box at the profile page of the person you want to follow.
Facebook is basically the same kind of service but there are a number of additional features, like a chat function, the possibility to upload pictures etc. Facebook also allows independent developers to develop applications for Facebook. These applications can be utilities of different kinds but often they are amusements like quizes.
Other services like LinkedIn and Plaxo which originally were more like directories where you could find people you know and make contact or have friends introduce you to other users you would like to get to know have now added a status field like the one Twitter and Facebook offers.
Myspace is a musically oriented community on the Internet which has all the characteristics of social media and has been around for a while and have many users. Other communities and services often mentioned in conjunction with social media are Gtalk, AIM, Plurk, Tumblr, Identi.ca, BrightKite, Friendfeed, Jaiku, Friendster, Flickr, Livejournal, Yammer and a growing number of services with some more or less unique feature or emphasis.