For class, we had to read "Got Facebook? Investigating What's Social About Social Media" which is a study by S. Craig Watkins and H. Erin Lee at the College of Communication (University of Texas-Austin).
Overall, the study was predictable as a regular Facebook user. The majority of people that use Facebook is college age adults (18 to 25) and the percentage users go down as the age goes up. Also, the majority of the study was done with Caucasians, which may bias the sample. The study also seemed to use college-educated people, this means the sample was most likely privileged, at least in class.
I was surprised at the average age of beginning Facebook, which was 22.69. Most of the people I knew started using Facebook during high school. So I thought it would be around 18. This was explained afterwards, saying that half of the people in the sample were introduced to Facebook at around high school age, but the other half were older. Another reasoning of this high age was the 'new' history of the website.