Facebook Inc. (Nasdaq: FB)should do more to inform users about its use of facial recognition technology and what it’s doing with its growing database of photos, U.S. Senator Al Franken said Wednesday.

Facebook, owner of the world’s largest social network with more than 900 million users, has facial-recognition software that suggest the names of friends that people can label, or “tag,” in photographs. It introduced the feature, called Tag Suggestions, in December 2010.

“Facebook may have created the world’s largest privately held database of face prints without the explicit knowledge of its users,” Franken, a Minnesota Democrat, said at a Senate Judiciary subcommittee hearing.

“Facebook could still do more to explain to its users how it uses facial recognition and to give them better choices about whether or not to participate in Tag Suggestions,” Franken said. “They could make clear to their users just how much data they have, and how they will and will not use their large and growing database of face prints.”

(The hearing came on the same day that YouTube announced technology to obscure facial identities in videos.)

Facebook has built privacy controls into Tag Suggestions, Rob Sherman, manager of privacy and public policy for Menlo Park, California-based Facebook, said at the hearing. The technology doesn’t let users identify people who are random strangers, and users can opt out of the photo-tagging feature, Sherman said.

The company also has safeguards for personal information it stores, including the templates used to power the tag suggestions, he said.

FBI Pilot

Franken also expressed concern about a Federal Bureau of Investigation facial-recognition pilot program that lets law enforcement officers take a photo of a person and compare it to a database of criminal suspects. The effort was started in Maryland, Michigan and Hawaii and will soon expand to three more states, Franken said.

“The FBI pilot could be abused to not only identify protesters at political events and rallies, but to target them for selective jailing and prosecution, stifling their First Amendment rights,” Franken said, saying any law-enforcement gains from the program could come at a high cost to civil liberties.

The FBI is deploying the system slowly to make sure there’s appropriate training and guidance in place, Jerome Pender, a deputy assistant director at the FBI’s criminal justice information-services division, said at the hearing.

More than 300 million photos were uploaded to Facebook on an average day during the three months ended March 31, according to the company. Facebook last month acquired Face.com, a facial- recognition startup and one of its technology suppliers.

“Our federal privacy laws are almost totally unprepared to deal with this technology,” Franken said. “Unlike what we have in place for wiretaps and other surveillance devices, there is no law regulating law enforcement use of facial recognition technology.”