Internet users tap Facebook Inc.'s "Like" and Twitter Inc.'s "Tweet" buttons to share content with friends. But these tools also let their makers collect data about the websites people are visiting.

These so-called social widgets, which appear atop stories on news sites or alongside products on retail sites, notify Facebook and Twitter that a person visited those sites even when users don't click on the buttons, according to a study done for The Wall Street Journal.

These widgets are prolific. They have been added to millions of web pages in the past year. Facebook's buttons appear on a third of the world's 1,000 most-visited websites, according to the study. Buttons from Twitter and Google Inc. appear on 20% and 25% of those sites, respectively.

The widgets, which were created to make it easy to share content with friends and to help websites attract visitors, are a potentially powerful way to track Internet users. They could link users' browsing habits to their social-networking profile, which often contains their name.

For example, Facebook or Twitter know when one of their members reads an article about filing for bankruptcy on MSNBC.com or goes to a blog about depression called Fighting the Darkness, even if the user doesn't click the "Like" or "Tweet" buttons on those sites.