At the recent ICWSM 2008 in Seattle I had an opportunity to present some thoughts about what makes social media social. The question is important because so many different types of interaction systems are considered social and they cannot all be the same thing. In an effort to categorize these systems one dimension that seems important is the size of the groups producing and consuming social media. In some cases on a single person creates an object that is then shared with many. In other cases many people create an object and then share it with just a few others. These dimensions open up a space in which different systems can be located and distinguished from one another.
A common misconception is that social media is authored by a large group (for example that many many people contribute to a Wikipedia page). In practice, I think most social media objects are authored by individuals or small groups and are intended to be consumed by small or large groups (most Wikipedia pages attract only a handful of people to make regular contributions). In aggregate, these objects become collective goods — what most of the great stuff on the Internet is made of: archives, collections, discussions, data sets, and more collectively authored by potentially millions of people.
The emerging sweet spot may be the creation of objects through the activity of large groups (think of the movements of whole financial markets, or the contributions of behavior from millions of search engine users) that are then consumed by large groups (in the form of market data or optimized search results).
There are other dimensions of social media beyond the size of the producer and consumer populations and I will try to post about some others. The nature of the digital object being produced or consumed is an important dimension, as is the level of interaction possible. Over time I would like to construct a taxonomy of social media systems so that we can clearly distinguish between email, wiki pages and massively multiplayer role playing games, all of which seem to be labeled “social media”.