The end could be near for cookies, the tiny pieces of code that marketers deploy on Web browsers to track people's online movements, serve targeted advertising and amass valuable user profiles.

In the past month, Microsoft Corp., Google Inc. and Facebook Inc. have said they are developing systems to plug into and control this river of data in ways that bypass the more than a thousand software companies that place cookies on websites.

The moves could radically shift the balance of power in the $120 billion global digital advertising industry—and have ad technology companies scrambling to figure out their next play.

"There is a Battle Royal brewing," says Scott Meyer, chief executive of Evidon Inc., a company that helps businesses keep track of the cookies on their websites. "Whoever controls access to all that data can charge rent for it—and has a tremendous advantage going forward."

The Silicon Valley trio, which produce browsers, email services and operating systems used by billions across many devices, are positioned to potentially learn far more about people's activities than cookies ever could. Today, a diverse ecosystem of companies places cookies on websites to track people through browsers; now the giants see an opportunity to get into tracking themselves.