Facebook used to be the domain of the college student. Then they expanded it to everyone but it was still mostly the domain of the young.
I remember last year when there was a 60 Minutes episode interviewing the creator of Facebook and Leslie Stahl created her own profile-- which she quickly deleted because she "couldn't understand it." I remember a story about Carl Kassell, the voice of "Wait Wait... Don't Tell Me!" on NPR having a Facebook profile and likely being the oldest person ever to be on Facebook (though it was maintained by his 20-something intern. Carl's on my Friend list).
I guess the college students grew up, got married, started having babies. Facebook was a way to put their babies' pictures up. Grandparents got involved. This past year I started seeing more and more gray hair on Facebook.
My mom is now on Facebook. Not only that, but some of her peer group and some even older than her have also gotten accounts and so she's finding friends quickly. Never ever thought I'd see it. It's just weird. Because Facebook is the place where college students upload drunken pictures of themselves that they would never show their parents, or their parents' friends. But now their parents and grandparents and extended family are all online and watching. It's an unnerving rapid shift in society.