Facial recognition allows surveillance agencies to identify a person's friends and associates in addition to identifying them, she said. She called on Congress to pass a law to regulate the use of facial recognition by law enforcement agencies.

At the hearing, Franken focused on an FBI pilot program in Maryland, Michigan and Hawaii and on a Facebook feature that tags pictures using facial recognition. The FBI and Facebook can serve as good examples to other organizations if they handle facial recognition technology appropriately, he said.

He called on Facebook to turn off its tag suggestion feature by default, instead of having it on by default, as it has in the past. But Rob Sherman, manager of privacy and public policy for the social-networking site, resisted that suggestion. Facebook has suspended the feature while it reworks it, but will bring it back soon, Sherman said.

On by default is appropriate, "because Facebook itself is an opt-in experience," Sherman said. "People choose to be on Facebook because they want to share with each other."

Facebook takes steps to limit the use of the facial data, he said. Only friends on Facebook can use the software to tag each other in photos, and Facebook users can prohibit the site from tagging them in photos, he said. In addition, the facial recognition templates are encrypted and only work with Facebook's proprietary software, limiting their usefulness to other organizations, he said.