Facebook’s pending initial public offering gives credence to the argument that personal data is the oil of the digital age. The company was built on a formula common to the technology industry: offer people a service, collect information about them as they use that service and use that information to sell advertising.

People have been willing to give away their data while the companies make money. But there is some momentum for the idea that personal data could function as a kind of online currency, to be cashed in directly or exchanged for other items of value. A number of start-ups allow people to take control — and perhaps profit from — the digital trails that they leave on the Internet.

“That marketplace does not exist right now, because consumers are not in on the game,” said Shane Green, who founded a company called Personal in 2009.

The idea behind Mr. Green’s company involves two steps. First, his team created a series of personal data vaults, which contain thousands of data points about its users (the company calls them owners). This data can be as prosaic as birth dates, or as specific as someone’s preference for spicy foods. People control what information they share and remove data they don’t want to share at any time.