My friend Paul has a dinky app on his phone. He's a bit of a geek so he thinks it's great. Other people think it's creepy.
This app is called Find my Friends, and that's exactly what it lets you do. On a little map, you can watch your friends' little blue dots sliding along little roads.
In the middle of the day, you can see where they're buying lunch. In the middle of the night you can see if they're in their own bed. And they can see where you are you, too.
Paul's boyfriend is a pilot. He told Paul this app was a bit stalky, but he secretly downloaded it anyway. Then, one day, after he piloted his plane to Hawaii, he had nothing to do.
Sitting all alone in his hotel room with only cable TV and tiny, tiny bottles of gin, he opened his creepy app out of sheer boredom.
Hoorah! One of his other pilot friends was in Honolulu, too. A phone call later they were out at dinner. That was the end of his boredom.
Anyway, that's the story Paul tells to convince you why this app isn't creepy.
He'll also tell you he once managed to squeeze in a quick shower before his friends arrived for dinner, because he could see they were still a 10-minute walk from his house. If that doesn't convince you, he has a back-up example about how he often avoids interrupting his mum at work, by watching for her to leave the office before calling.
Paul's in his 20s, so he has grown up with a mobile glued to his hand. He doesn't know a world where he and his friend need to be near landlines at the same moment in time to talk on the phone.
So sharing his GPS location doesn't feel uncomfortable. He's sharing only a tiny bit more information now than he used to on Facebook.
And on Facebook, he was only sharing a tiny bit more information than he used to on MSN Messenger.
And so, he has reached this point of zero privacy one tiny step after the other.
This is the age of social media. We share everything we see and do. We can't take it back. We'll have as much luck opposing it as our grandparents had opposing television.
It's here to stay, and it's up to us to figure out how to use it.
It has been a hard lesson for some.
The man in France who filmed the Charlie Hebdo terrorists murdering a policeman in the street outside his apartment put the video on Facebook.
It took him 15 minutes to regret that. By the time he pulled the video off his page, the TV news networks had it.
Maybe the Cantabrians who uploaded video evidence of the romp in the office across the road also regret it now. Maybe they don't.
Either way, they have got us asking ourselves whether we'd do it. Upload the video, that is.
Social media has turned our world into a little country village, where everyone knows everyone else's business, thanks to party line telephones and the local gossip.
Nowadays, though, you don't just know the business of a hundred people. You know the business of a hundred thousand.
I didn't tell my friend Paul, but that app turned out badly for a lady in New York. Her husband sneakily loaded it on to her phone and found out she wasn't at her friend's house when she said she was.
She was cheating.
The age of information-sharing is brilliant, as long as you have no secrets.