I did a quick analysis of the last 30 years of my High School and students/former students participation in Facebook. Here is the graph:
The accumulated 70’s and 80’s graduates only amount to 80 people.
The 90’s graduates are less than 500 strong.
The 21st century graduates are almost 2000 and the decade is not even over. There is no single inflection point, but 97 had 46 people and then 98 had 88. 99′ had 93 people and 2000 had 136. Those are the only years where, on a percentage basis, “interesting” growth happened (by that, I mean absolute growth of more than 15 people (so I rule out the years when it went from 3 to 9 or 1 to 5) and percentage growth greater than 30% (91% and 46% respectively))
So this population skews pretty young. We are talking about high school here, so ~75% of the people on Facebook that attended my high school are 25 or under.
My year (Class of 91 – wooooo) had a graduating class of more than 700 and only 27 are on Facebook. Facebook’s efforts to target an older population and business crowd are just going ok, I would say.
It is a shame I don’t have data about when they joined so I could look at how this curve is changing over time. Maybe I should check back in a month or quarter.
Update: Thomas makes a great point that you could compare this to demo data and actually try to build a model of when people stop identifying with their high school – and this might be a better/more accurate thing to do. He points to a TechCrunch link that shows some basic demo data for Facebook that indicates that ~50% of Facebook members are over 25. Pretty interesting! I still want to go back and run these numbers in a month or so and see how they change.