With this action, Twitter becomes an official supporter of the voluntary privacy initiative for U.S. companies, which has been backed by the Federal Trade Commission, the Mozilla Foundation and online privacy organizations. A set of best practices for U.S. sites goes into effect next year. In Europe, the mandatory European Union Privacy Directive goes into effect on May 26 for all European-based companies and for multinationals.

'Tailored Suggestions'

Twitter has begun implementing experiments in "tailored suggestions," which recommends whom users might want to follow, based on a user's personalized information. The site points out on its company blog that, as a supporter of Do Not Track, "we will not collect the information that enables this feature" if someone has DNT enabled in browser settings.

Twitter has gone so far as to tweet that "we applaud the FTC's leadership on DNT." The move by Twitter was praised Thursday in a statement by Sen. John Kerry, D-Mass., who said the action "is something that responsible, competitive companies can do." Mozilla has recently noted that nearly 9 percent of its desktop users and 19 percent of its mobile users are using the DNT feature in its browser.

A Do Not Track option is now available in Mozilla's Firefox, Microsoft 's Internet Explorer and Apple's Safari Web browsers, and soon in Google's Chrome. But in the U.S., Web sites have the option to comply or not.

To assist Web sites in managing and disabling tracking-related third-party tags, Cupertino, Calif.-based Ensighten recently released a free Web tool. Called PrivacyDNT, it allows Web site owners to identify all third-party tags, create lists of which ones should be blocked in general, and keep specific tags from being activated during a session for a visitor who has enabled DNT in the browser, among other functions.

'High Level of Anxiety'

Tracking technology, including cookies, results in user profiles that are then used, and sometimes sold or exchanged, extensively in the online advertising and marketing industries. The technology also has benefits for users, in that it makes possible functions like shopping carts and personalized information or ads.

Andrew Frank, research director for Gartner , said that the advertising industry has "a pretty high level of anxiety" about the adoption of DNT, with many companies reviewing their policies and considering contingencies.

A great deal of the anxiety, Frank said, is due as much to the vagueness of the standards as to the consequences of users turning off tracking. He said that even with Europe's tougher standards, there are many questions about how to obtain user consent, what is permitted and other issues.

Some possible directions at this point, Frank said, include the possibility that the ad industry will be "more explicit about exchanging value for tracking," perhaps offering incentives to do so. There might also be different levels of tracking, so that individuals agree to some levels but not others.

In any case, Frank said, "I don't think it's likely that tracking will end soon because no one will consent."