Millions of people gave their email addresses to Quibi, JetBlue, Wish and other companies — and those email addresses got away.

They ended up in the hands of advertising and analytics companies like Google, Facebook and Twitter, leaving the people with those email addresses more easily targeted by advertisers and able to be tracked by companies that study shopping behavior, according to a report published on Wednesday.

The customers unwittingly exposed their email addresses when signing up for apps or clicking on links in marketing emails, said the researcher Zach Edwards, who runs the digital strategy firm Victory Medium. In the report, he described the giveaway of personal data as part of a “sloppy and dangerous growth hack.”

The practice of making customers vulnerable to tracking by allowing their personal data to be passively collected by third parties is nothing new, Mr. Edwards said in an interview, but it has gained traction despite efforts to improve online privacy protections.