SAN FRANCISCO - Google's popularity was built on its ability to help people find just the right Web pages. Then came the social Web, led by Facebook, where people go to see vast amounts of material that has largely been off-limits to Google - conversations and photos posted by their friends. On Tuesday, Google sparked controversy when it made some of the biggest changes ever to its search results, adding content from its fledgling Google Plus social network. That includes posts, photos, profiles and conversations from Google Plus that are public or were shared privately with the person searching. With the new feature, which Google calls Search Plus Your World, it is once again reimagining what people want from its search engine, which for many is the front door to the Web. It is betting that they are looking for more personalized and timely updates than it has been offering, as it competes with Facebook and others for users" time. "What you search today is largely written by people you don't know; we call that the faceless Web," said Amit Singhal, a Google Fellow who oversees search. But many people turn to Google for links to expert information, and might not want their search results to be cluttered with friends" postings, or find it disconcerting to see personal information from friends in search results. For some experts the new feature also raised antitrust concerns, because in many instances Google will show information from its social network before or instead of others. Twitter said it was concerned that Google's change would make timely information from Twitter harder to find on Google. "We think that's bad for people, publishers, news organizations and Twitter users," the company said in a statement. Danny Sullivan, an expert on the search industry and editor of the blog Search Engine Land, said Google was right that people wanted to see social posts - but most of those posts are on other social networks. "A lot of it is not on Google Plus," he said. Google's job should be, what are the best social accounts I should be following, not the best Google Plus accounts I should be following. But the changes show how important Google Plus is to the company. By failing to get on board with social networking sooner, Google risked being left behind. Search Plus Your World comes eight years after Facebook started and in the weeks before Facebook is expected to file for an initial public offering, which is likely to be the crowning moment for the new social Web. To keep up, Larry Page, Google's chief executive, prioritized social features and introduced Google Plus last summer, after fumbles like a social network called Buzz and tense volleys with Facebook, which does not allow Google to include most of its pages in search results. Google has said it does not want to replace Facebook. Rather, the company wants to have a service that supplies it with social information that it can use in its other products when users are logged into a Google service like Gmail. For instance, for most users, a search for "chikoo" would show links about and photos of an Indian fruit. But for friends of Mr. Singhal, it would also show photos and posts about his dog, Chikoo. A search for a sports team would show, in addition to the usual links, conversations about the team among a user's friends on Google Plus. When people search for a name, Google will highlight people who are friends with the searcher on Google Plus, or prominent people. And in searches for general topics, like "cooking," Google will show Google Plus profiles of celebrity chefs on the right side of search results. These features raised red flags for some antitrust experts because Google, which is under investigation by federal regulators over whether it favors its products in search results, could show a chef's Google Plus profile above her Facebook or Twitter profile or her own Web site, for instance. "When Google shifts towards just searching itself, and it directs you away from stuff on the rest of the Web and other social networks, people are forced to stay inside the Google ecosystem," said James Grimmelmann, an associate professor at New York Law School who specializes in Internet law. It reduces the conversations that everyone can be a part of across the Web. Google said that its search engine already showed some material from sites like Flickr and Twitter when users link their accounts to their Google profile, and that it would include material from sites like Facebook if the companies would make it available. "We want to help you find the most relevant information from your friends and social connections, no matter what site it's on," Google said in a statement. However, Google does not currently have access to fully crawl the content on some sites, so it's not possible for us to surface all that information. Ushering in the new era of social and private data search will take close cooperation, and we hope other sites participate. Facebook, which has little incentive to open its pages to Google searchers, declined to comment. Google users can click a link on a page of search results to see only personal posts, or turn off the new feature and see only standard search results. Users who have not signed up for Google Plus will see any other items they have shared with Google, like photos they have uploaded to its Picasa service, and items posted publicly on Google Plus by people Google assumes they know because they communicate with them on Gmail, for instance. Like many social networks, Google Plus has its share of spamlike comments. Mr. Singhal said Google had created algorithms that show only the most relevant posts in search results. For example, it guesses how close a user is to a friend on Google Plus based on how often they communicate. "Our job is to provide relevant suggestions, and just because someone is discussing something on Google Plus, if it's not prominent enough, we don't want to bring it to the search results page," Mr. Singhal said. Nick Bilton contributed reporting.