The second part of face recognition involves matching unknown faces to those already identified. This relies on both math and the way that people take photos. For instance, it's common for a user to take 10 or more photos at a party, and for the same people to show up multiple times. Riya takes advantage photo groupings to supplement its face analysis algorithms, Gokturk says.

In addition to scanning photos for faces, the software also notes characteristics of clothes, such as color, texture, and shape, and links those with faces. "At an event, you rarely change the shirt you're wearing," says Gokturk.

The software also looks for pictures with people together, such as a husband and wife, along with the date the photo was taken, to help the software take shortcuts in identifying people. For instance, if an individual is facing directly at the camera in one photo, but turned slightly, hiding some features, in another taken a second later, it's still likely that it's the same person.

By integrating such clues, Riya has developed face recognition software that may not be the most accurate, but can do the job with a degree of accuracy that could be "good enough" for consumers, says Stan Bileschi, a computer vision researcher at MIT.

Once the software is trained -- Riya recommends uploading 500 photos initially -- its able to correctly identify people 80 percent of the time, Gokturk says. He adds that the company is constantly improving its product, as well as exploiting social-networking capabilities of the site to improve accuracy. For instance, if your friends have already identified people in pictures, you can use their tagged information to identify common friends.

Given all the headline news recently about the National Security Agency amassing a database of information about U.S. citizens (or at least phone calls), privacy issues are highly visible right now. And with facial recognition tools becoming available, some worry that unethical individuals could use them to carry out crimes such as identity theft or cyber-stalking. "This is a legitimate concern -- but face recognition technology like Riya should be the least of their worries," says MIT's Bileschi. "Remember that every time you do something on the Internet, like search, every time you make a purchase in any form other than cash, and every time you use your fast-lane pass to get on the highway faster, your individual presence is being recorded...Face recognition technology is not nearly as accurate or as pervasive as these forms of personal tagging."

Additionally, Riya has rules that restrict access to photos. For example, you can decide if you want photos to be viewable publicly or only to you and some designated others. Or worried about being recognized in the background of a stranger's photo? Riya's software only tags people that one has trained it to recognize.

It's important that companies offering this technology establish rules and restrictions on its use, says Carnegie Mellon's Lowe. In the end, the concern may not stem from a particular application, however, but from the management of all the online databases with photos. Currently, Mellon estimates, most people know if they are in these databases. But with more online repositories storing photos and other personal information, he says, "everyone needs to be more vigilant about managing the data that's out there."