With about half of Facebook’s 400 million users checking in daily, the social networking company has established itself as one of the Web’s most popular destinations. Now Facebook is intensifying its efforts to expand its empire beyond its Web site; the company wants to turn scores of sites across the Internet into satellites where users will be able to interact with their Facebook friends. Details of Facebook’s plans — which involve a variation on its “Share” button, already prevalent on many sites — are expected to be introduced by Mark Zuckerberg, the company’s chief executive, on Wednesday during its conference for third-party developers. But even before Facebook makes its plans public, its aim to become a social networking force across the Web is facing competition. On Monday, a coalition of other companies, including some Facebook rivals like Google and MySpace, are banding together to establish a new standard for Web sites to allow visitors to share information, not just with Facebook but also with dozens of other social networking sites. The coalition is led by Meebo, a company that offers a toolbar featured at the bottom of many Web sites that visitors can use to share articles, photos and other links with a variety of social networking services. In the meantime, Twitter is also looking to expand its presence across the Web with its @anywhere service, which will allow people to log in to Twitter from other Web sites. The moves by Facebook and its rivals set up a battle for control over social interactions across the Internet.