One of the questions I'm asked often when I speak at conferences is which social media platforms should our organization invest staff time and resources in? My response is "where is your community hanging out." This requires organizations to research the answer since every community has different preferences.
One good place to start looking at this data is to take a look at the demographics of different social networks. Realtime Report does a great roundup of the latest social media stats every couple of weeks. I also found this infographic that provides an overview of some useful stats to check out.