I have never, never in my life been anywhere near Facebook. I wasn't especially surprised to get two invitations recently to befriend certain people on Facebook. What did suprise me was the rest of the message which listed various other people on Facebook who I supposedly knew.
This means that entirely without action on my part, Facebook has a pretty good profile of the range of people I have contact with in various ways - a bunch of poets, some distant relatives I've contacted on genealogy matters, people I've worked with on various committees, and so on.
How does this happen? I had to ask my offspring to explain it to me. Apparently when you join Facebook, you can tick a box and Facebook will rifle through your e-mail address list, store the addresses, and check to see if any of them are also on Facebook. Which may seem like a good idea at the time, but it means that you are giving out information that your contacts may well have considered private, to be stored by Facebook forever plus a hundred years or so.
It all makes me rather uneasy.