You have a great picture of your kid on their first day of school and you would like to share it with just two people. You could post to Facebook or Instagram, but then everyone sees it. You could attach it to an email or text or you could just pass the phone over the next time you see them.
The company XY has another option. It has designed a Bluetooth device the size of a keychain that lets you share an album of pictures with select people.
