Facebook has been criticized for its data-handling in the past. In 2011, the US Federal Trade Commission (FTC) said Facebook engaged in "unfair and deceptive practices" by publicizing data that users had thought was private. It signed a "consent decree" with the FTC, agreeing not to share users' data without their express consent. If the FTC finds that Cambridge Analytica acquired the data of 50 million users without their consent, this would be a major breach of that decree Facebook signed, potentially resulting in massive fines for Facebook, and regulation that could affect the entire digital advertising industry. But any app or service you've signed into with Facebook could potentially be doing something similar.