Google on Tuesday said it will begin tracking people as they use Google search and Gmail, watch YouTube videos and use other Google services -- at their computers and on their mobile devices.

That announcement came hours after Facebook said it will make Timeline and Open Graph part of the default user interface. These new services chronologically assemble and make more easily accessible the preferences, acquaintances and activities of its 800 million members.

The two changes will become effective over the next few weeks, the tech giants said. "It's clear that they're doing this to chase more advertising revenue," says P.J. McNealy, analyst at Digital World Research.

The $32 billion U.S. market for online advertising in 2011, as estimated by eMarketer, is expected to grow at a robust clip. Apple, Microsoft, Twitter, Yahoo, Adobe and a flock of Internet-based companies are part of this gold rush.

Google and Facebook have been sanctioned by U.S. regulators for privacy violations. The moves disclosed on Tuesday are sure to draw scrutiny.

"Google's plan to change its privacy policy raises important questions about how much control Google users will have over their personal information," said Rep. Ed Markey, D-Mass. Critics worry the tech giants will open fresh opportunities for cybercrooks to prey on users of the sites.

"Both are racing to monetize our private information and in doing so creating collateral damage," says Alisdair Faulkner, chief product officer at security firm ThreatMetrix. "They are essentially indexing more and more private information and, in doing so, serving it up on a platter to cybercriminals."

Previously, any trace of a user's search or surfing activities on a Google product was used to generate ads only on that particular product. After March 1, when the new policy change becomes effective, a Google account holder's search and surfing activities will be tracked and used to...