Is this the beginning of a backlash? It looks as though there's some research out there to suggest that the over-55s are starting to desert Facebook.
There could be a number of reasons for this. The best guesses I've seen mostly involve money. This time last year you might recall there was a slew of stories about people being banned from social networking at work because - wait for it - working time is for working, and if you want to do social stuff then the polite thing seems to be to do it on your own time. (Just say if I'm going too quickly for you).
It looks as though a lot of people are starting to work this out for themselves too. If they spend all their time socialising then guess what, their earnings might go down. Freelances will explain to anyone who'd care to listen that we don't get paid for our time but for our output. My guess is that a whole lot of people heard social networking was doing great things for a load of others, got involved, found it wasn't productive for them and their clients, and have now gone away.
Other explanations include the idea that the older people find the real-time pace a little too much. I've seen that said on a number of websites. Personally at 44 I'm already old enough to recognise patronising drivel when I see it, as I was saying to my 88 year old Aunt on Facebook only the other day. But a lot of people seem to be thinking it.