The act already requires Web companies to notify parents and obtain consent when personal information is collected from children under the age of 13. But the law, which took effect in 2000 — four years before the start-up of Facebook — did not envision the now common practice of including add-ons on children’s sites.

As written, the privacy act applies to a Web site operator, but not to a third-party supplier of other functions, said Mary K. Engle, associate director of advertising practices at the F.T.C. “The proposal would close an apparent or possible loophole in the rule,” she said.

Other tweaks in the proposed rule would address sites that are used by both children and adults. Where the current rule treats all visitors as being under 13, the proposed rule would allow a Web site that attracts both children and adults to screen users by asking their age and applying the privacy protections only to those who say they are under 13.

That kind of self-reporting can be manipulated. Facebook, for example, currently requires a user to be at least 13, but many children misrepresent their age, often with parental permission.

In a comment to the commission addressing the changes proposed last year, Facebook said that it had tiered screening, including “technical checks at sign-up, social verifications and reports from our community to help identify child accounts,” which were deleted once they are identified.