Facebook Inc. introduced an e-mail service for users of its social-networking site, stepping up competition with Google Inc., Yahoo! Inc. and Microsoft Corp. Facebook is giving all users an "@facebook.com" e-mail address, Mark Zuckerberg, the company's chief executive officer, said today at an event in San Francisco. The new service will include text messaging and instant messaging as well as traditional e-mail. By adding e-mail, Palo Alto, California-based Facebook provides an alternative to Google's Gmail, the fastest-growing Web mail service in the past year. It also steps up pressure on Microsoft and Yahoo, which are vying for the attention of Web users, who rely more on social sites for information and staying in touch with friends.