Willie Sutton said he robbed banks because that's where the money is.
For organizations wondering why to get involved in social media, the reason to do so is that that's where the people are.
Fewer of them are watching broadcast TV...certainly on a television set. There are people watching programs via their computer but that seems to be a one-to-one experience as opposed to watching TV sets, where it's more likely that other people will be there, too.
Far fewer of them are reading print newspapers and magazines.
They're listening to radio primarily during drive-time. And in a few years, they won't even be doing that. (The current generation of kids don't listen to the radio. Some estimate that radio's audience will fade away in five years. I don't necessarily agree, but I do see that my own kids never listen to the radio at home -- in fact, we only have two radios at home, and one's in my office, so it's not really convenient for them -- and they don't listen often in the car. It's just not going to develop into habit for them.)
Instead, more people are online, reading newspapers and magazines (mostly for free), listening to music, watching television shows...and interacting with other people via social media.
According to a recent BrandWeek article, "MySpace Suffers as Time on Facebook, Twitter Rises," "Social networking as a whole has been on a roll, according to Nielsen figures. Total time spent on social networking sites increased 81 percent compared to a year earlier."
If you're not part of it, you're missing out.