The social networking Web site Facebook signed an agreement with 48 states and the District of Columbia yesterday that Pennsylvania Attorney General Tom Corbett believes will make the site safer for underage users.

Under the agreement, Facebook will join the Internet Safety Technical Task Force, an industrywide group dedicated to developing age and identity verification for online users. The agreement also included the following provisions:

• Age locking, which will prevent members who initially register as being 18 and older from changing their age to being younger than 18 unless the change is reviewed by a customer service representative.

• Allowing minors to block members 18 and older from viewing their profiles.

• Disallowing members 18 and older from performing age-based searches for members younger than 18.

• Preventing registered sex offenders from being on Facebook.

• Targeting tobacco and alcohol advertisements, if accepted, only to users who are legal to buy the product.

• Maintaining a list of pornographic Web sites and immediately severing any links from Facebook pages to those sites.

The agreement is modeled after one that MySpace, the world's largest social networking site, signed with attorneys general across the country in January.

Sexual predators had used the site to troll for underage victims, sometimes soliciting them online.

Eric Shirk, a spokesman for the Pennsylvania attorney general's office, said he did not know of any such cases on Facebook, but called the agreement a "proactive approach."