Sites such as Quantcast and Alexa provide basic demographic information compared to the overall Internet population, as shown here. Quantcast also displays the distribution by subcategory within the site. Alexa's free version now provides only limited information, although it does offer a seven-day free trial. As you can see, the sites don't always share the same subcategory breakdowns or completely agree on the data. However, either one is close enough for your social marketing purposes.
Quantcast (top) and Alexa (bottom) provide demographic profiles comparing users of a site (in this case, LinkedIn) with the general Internet population.
Use these tools to check out the demographic profile of users on various social media services, as well as your own users and those of your competitors. For instance, by comparing the demographics on Quantcast, you can see that LinkedIn appeals to an audience that is older, more male-dominated, and better educated than visitors to Facebook.
Look for a general match between your target audience and that of the social media service you're considering.