The app has been developed by face.com which is said to be working on Facebook's facial recognition programme.
Called KLIK, the app is currently in the Apple app store and it is free.
At this stage, the app is restricted, for privacy reasons, for showing you information about people with whom you are connected on Facebook.
However, it is unlikely that this restriction will endure for long, now that face.com has made its software, called Photo Finder, available to developers who are building applications which let you search for anyone whose photo is on the internet.
'The technology is already being used by 5,000 developers," says Gil Hirsch, founder and CEO of face.com, "you can basically search for people in any photo - on Flickr, in newspapers, or in videos on YouTube - but it would take a lot of processing power."
Presumably, in a handset, the processing would have to be done in the Cloud
The software reduces the measurements and characteristics of a face to an algorithm which can be compared to any photo published on the internet. Face.com says it is 90% accurate.
UK customs is currently using face recognition technology to recognise people coming into the country at airports.
The privacy issues are, of course, horrendous but, once something becomes possible, it is unlikely that it's going to go away.