The Federal Trade Commission said on Thursday it found that several social media and streaming services engaged in a “vast surveillance” of consumers, including minors, collecting and sharing more personal information than most users realized.

The findings come from a study of how nine companies — including Meta, YouTube and TikTok — collected and used consumer data. The sites, which mostly offer free services, profited off the data by feeding it into advertising that targets specific users by demographics, according to the report. The companies also failed to protect users, especially children and teens.

The F.T.C. said it began its study nearly four years ago to offer the first holistic look into the opaque business practices of some of the biggest online platforms that have created multibillion-dollar ad businesses using consumer data. The agency said the report showed the need for federal privacy legislation and restrictions on how companies collect and use data.

“Surveillance practices can endanger people’s privacy, threaten their freedoms, and expose them to a host of harms, from identify theft to stalking,” said Lina Kahn, the F.T.C.’s chair, in a statement.

Tech giants are under intense scrutiny for abuses of privacy and have in recent years been blamed in part for contributing to a mental health crisis among young people and children that has been linked by some social scientists and the surgeon general to the rampant use of social media and smartphones. But despite multiple proposals in Congress for stricter privacy and children’s online safety protections, nearly all legislative attempts at regulating Big Tech have failed.

Efforts by the companies to police themselves also haven’t worked, the F.T.C. concluded in its report. “Self-regulation has been a failure,” it added.