I really like Facebook's take on private photo sharing, which uses scary-good facial recognition technology.
Moments, an app launching today on Android and iOS, attempts to rescue the hundreds of photos on your camera roll that feature your friends and let you share them with a few taps. Using the face-recognition technology that powers the suggested tags you see when adding a photo of a friend, Moments scans your camera roll for familiar faces and lets you quickly "sync" them to the subject of the photo. If your friend uses the app, they will see the photos added to their own synced collection. If they don’t, they’ll get a notification on Facebook Messenger that photos are waiting for them in Moments. The app then builds a collection of photo albums featuring you and your tagged friends that you can scroll through, search, and easily rename.