Mostly this came about from a thought experiment in guessing what information they can figure out about you based on your profile. Do I honestly think that there is a whole team of engineers sitting around charting out menstrual cycles for 50% of their users? Nope. Is there a nonzero chance that it might be happening or could happen soon? Yeah I think so. My main beef is that facebook can infer a ton of information about me that I don't want them to know and that they could someday be selling to people who I really don't want to know that information.
Also 6 billion people don't have a Facebook account, but condition on Americans between the ages of 18 and 30, and the rate of account ownership will probably go way way up. Those are the people who always ask me why I don't have one. Almost everyone I know has one and I really do have to explain once a week why I don't have one and don't want them to make one for me.