This chart from recent Ofcom research is up on Techcrunch today. It shows pretty clearly that an increasing number of people are social networking across all ages, with an especially large increase amongst those age 55+. I imagine this activity is largely on Facebook, with maybe a little on Twitter.
One intesesting thing to note is the length of time it takes older folks to pick up on tech trends that are popular with kids. Facebook was founded in 2004 and at that point social networking was already pretty widespread amongst students, so in this case it took eight years for the new technology to reach 25%+ penetration across all age groups.