Since the heyday of AOL's cheery "You've got mail" greeting, e-mail has been central to the online experience for millions of people.
But Mark Zuckerberg, the chief executive of Facebook, says e-mail is showing its age.
In his view, e-mail is too slow, too formal and too cumbersome, especially for young people who have grown up using text messages and online chats.
On Monday, Mr. Zuckerberg introduced a new unified messaging system for Facebook that allows people to communicate with one another on the Web and on mobile phones regardless of whether they are using e-mail, text messages or online chat services.
"We don't think a modern messaging system is going to be e-mail," he said.
The new service, Facebook Messages, is a bold move by Facebook to expand from a social network into a full-fledged communications system. It could help the company chip away even more at Internet portals like Google, Yahoo, MSN and AOL, which have used e-mail as one of their main draws with consumers.
Americans already spend more time on Facebook than on any other Web site, and more than 500 million people around the world have signed up for it.
Analysts say that if Facebook Messages proves successful, it could greatly increase the time users spend on the site, making Facebook even more dominant.
Some analysts say, however, that the company will face a number of challenges, like managing spam, getting users to change ingrained habits and persuading some to entrust their confidential e-mail to a company whose privacy practices have often drawn scrutiny.
What's more, Mr. Zuckerberg, while successful in turning Facebook into a behemoth, has at times failed to accurately gauge technology trends. In 2007, he described a new advertising system released by Facebook as a "once-in-a-century" shift in media, but was forced to pull the plug on the service amid a storm of privacy complaints. That incident and others suggest he tends to overestimate how open people want to be with their personal information.
The new service, which will encourage users to sign up for an e-mail address ending in @facebook.com, has the immediacy of instant messaging and chat built in. Mr. Zuckerberg sought to play down the threat that Facebook Messages would pose to existing e-mail services.
"This is not an e-mail killer," Mr. Zuckerberg said, adding: "We don't expect anyone to wake up tomorrow and say they are going to shut down" their current e-mail accounts.
In addition to channeling all e-mails, text messages and chats through a single point, Facebook Messages will offer users what Mr. Zuckerberg called a "social in-box" that will prioritize messages from friends and close acquaintances, potentially saving time. And it will make it easy for people to retrieve all the communications they've had with a person through various channels. The service is invitation-only for now, and will be introduced to all users over the next few months.
Some analysts said that over time users were likely to spend more time using Facebook Messages and less with their traditional e-mail services, especially as they communicate with their closest friends and associates.
"They just made it so much more compelling to center my communications on Facebook rather than anywhere else," said Charlene Li, an analyst with the Altimeter Group. "Google, Microsoft, Yahoo should all be worried."
Ms. Li said that e-mail was already being "nibbled to death" by services like instant messaging and chat, and that Facebook Messages, if successful, would accelerate that trend.
Still, for more than a decade, technology companies have sought to offer services for "unified communications," often without much success outside of the business market. And other e-mail providers, including AOL, Google and Yahoo, have taken steps to make their e-mail services more "social," by prioritizing the messages of friends or integrating text messages.
"Just like it is not easy for traditional e-mail companies to compete in social, it is not going to be easy for social companies to compete with e-mail," said Brad Garlinghouse, president for consumer applications at AOL, which on Sunday unveiled a service that also allows consumers to consolidate e-mail and other messaging accounts in one place.
Mr. Garlinghouse said that, for example, Facebook's efforts to use connections between users to prioritize their incoming mail might be fraught with peril.
"I am not friends on Facebook with my accountant, with my doctor, or with United Airlines," he said, but messages from any one of those sources could be urgent.
Kakul Srivastava, a vice president in charge of communications products at Yahoo, said, "Integrated communications is definitely something that people are looking for."
Ms. Srivastava says Yahoo users already send 2.5 billion instant messages a month and four million to five million text messages a day using Yahoo Mail.
Mr. Zuckerberg, who is 26, said the idea for the service came out of conversations he had with teenagers nearly two years ago. While the teenagers said they all had e-mail accounts, they told Mr. Zuckerberg that they did not use them very often.
"Talking with high-schoolers makes me feel really old," Mr. Zuckerberg joked.
Mr. Zuckerberg says 350 million Facebook users already use its messaging service, exchanging more than four billion messages each day.
The new Messages product will not require users to get a facebook.com address, but users who do not will not be able to receive messages from outside Facebook.
The 15-month effort to develop Messages was the biggest engineering project that Facebook had undertaken, involving 15 engineers, Mr. Zuckerberg said.