Royal Pingdom conducted a study of the age distribution of different social networking sites. It's often middle-aged people who dominate the user bases of these sites:
Although we can’t say how this will change over time, at the moment the older generations are for one reason or another (tech savvy, interest, etc.) not using social networking sites to a large extent. This probably reflects general internet usage, but we suspect the difference is enhanced when it comes to the social media sphere where site usage tends to be more frequent and time-consuming than usual.
It is also noteworthy that social media isn’t dominated by the youngest, often most tech-savvy generations, but rather by what has to be referred to as middle-aged people (although at the younger end of that spectrum).
More charts at the link.
Link via The Presurfer