The US government is gearing up to investigate whether Amazon, Apple, Facebook and Google misuse their massive market power.

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When users sign in with their Google or Facebook Inc profiles to third-party apps, the apps often share valuable data with Google and Facebook, a practice that Apple is looking to stop.

Apple, beset by falling iPhone sales, announced upcoming changes to its phone and computer software intended to highlight its increasing emphasis on digital services and to further position it as a fierce guardian of personal privacy. The revisions previewed Monday during a conference in San Jose, California, included a new feature that will let people log into apps and other services with an Apple ID instead of relying on similar sign-in options from Facebook and Google — two companies that mine data to sell advertising. Apple said it won't collect tracking information about users from that service. As part of that feature, Apple will also let users mask their true email addresses when signing into apps and services. That will involve faux email addresses that automatically forward to the user's personal email. When the next version of the iPhone software comes out this fall, Apple is also promising to give people the option of limiting the time apps can follow their locations and prevent tracking through Bluetooth and Wi-Fi signals. The revisions are part of Apple's ongoing attempts to differentiate itself from other technology giants, many of whom offer free services in exchange for personal data such as whereabouts and personal interests, which in turn fuels the advertising that generates most of their revenue. Apple, by contrast, makes virtually all its money selling devices and services, making it easier for CEO Tim Cook to embrace "privacy is a fundamental human right" as one of the company's battle cries in an age of increasingly intrusive technology.