Virtual Privacy

Yesterday, a current student told me that a former student was a Kennedy. So tonight, bored, I checked it out, and it's true. There are a number of prominent families at our school, so I looked them up to see if their families were famous in any way. What I found instead bothered me greatly. I googled the last name, and a facebook group page came up. It contained the cell phone numbers of many of my former and current students! They are smart enough to make their profiles private, but when a friend makes a public group because he lost his cell phone, they all posted their numbers for the world to see! Should I tell the administration? Warn the kids I know?