Facebook CEO Mark Zuckerberg makes the announcement. (Screenshot from live feed)

Facebook announced Monday morning that its 500 million users will receive @facebook.com e-mail addresses and a new "social inbox," demonstrating an effort to maximize the amount of time its users spend within's Facebook walls.

Facebook CEO Mark Zuckerberg said about 350 million Facebook users regularly use the website's messaging service. About 4 billion messages or instant messages are sent daily. But there's a lot of confusion, extra work and spam that comes with messaging and sending e-mails. Zuckerberg said high-schoolers told him they don't like the formality of e-mails--from filling out subject lines to attaching signatures. Facebook's new "social" inbox also makes it tougher to receive unwanted contact.

"This is not an email killer, this is a messaging system that includes email as part of it," Zuckerberg said. "We don't expect people to turn off their Yahoo account or their Gmail account and use this.

"It's like those high school students, they're shifting to real-time messaging because it's simpler and helps people connect better and it's more fun and more valuable for them to use. If we do a good job with that, in six months, a year, two years out people will say this is the way to do it, maybe email isn't as important a part as it was before, and we can push messaging towards this important real-time simpler experience we can."

He said the next generation of messaging would be "seamless, informal, immediate, personal, simple, minimal, short."

Zuckerberg's vision includes intertwined messaging (e-mailing as quick and fast and text messaging and everything going into one thread), conversation history (an ability to look back on information that comes from someone through any platform all in one stream) and a social inbox (in which Facebook is able to use your friends and networks to make sure you only see the messages that you want to see). The service combines instant messaging, e-mailing and text messaging.

"It will feel like an ongoing conversation rather than fragmented messages that don't relate to each other," said Andrew Bosworth, Facebook's director of engineering.

However, there's still many questions that won't be answered until users can start messing around with the system. It's unclear how group messages or event messages will fit in; how messages sent to multiple users will fit into the conversation history; or how simple it will be to contact people who are not on Facebook.

The social inbox rivals Google's priority inbox on its Gmail service. Facebook's e-mail service will also have a priority inbox with those messages that its filter find most important to the user and a separate inbox with items that don't pass through filters quite as readily. A third folder will simply hold junk.

Facebook's default filter is only accepting messages from the friends of friends network. All other messages, including those from people not in your network will go to the other folder. People can promote messages and contacts to the main inbox. Messages from friends will always have high priority--no matter the acual content or substance of the individual message. As one person at the announcement put it, an e-mail from your mom with funny quotes will receive just as much love as an e-mail telling you to pick up her medication.

Messages sent to the e-mail address will work like a chat with users able to respond through a chat box or the messaging service. But messages will be sent to the platform where it began. If someone sends you an e-mail message, it will first come through as an e-mail. The wider idea returns back to the "seamless intergration" vision. The new service essentially blurs the lines between an instant message and an e-mail. Everything will go into a single thread, eliminating multiple very long e-mail threads and bringing all chains with the same person to one place.

The new service launches later Monday, including on Facebook's iPhone application, but users may not see it in their account for a few months as Facebook offers gradually invites users to participate. The service was designed by 15 engineers--the most Facebook has ever dedicated to one project.

Users will be able to decline accepting a @facebook.com e-mail address and still use the revamped messaging service. Users will also be able to promote or demote friends to receive more or fewer messages from certain friends. They can also block certain friends or choose to accept messages from only friends. The system also supports attachments.

The inbox will feature advertising on the right-hand side that is no different than the ads already on Facebook pages.

Messages can be deleted or archived, but by default everything will be saved no matter which platform it is sent or received through. Messages can also be forwarded. Facebook says users will not run into inbox space limits under normal use, though they declined to provide an exact limit.

The service also integrates Microsoft web apps, meaning users will be able to Word, Powerpoint and Excel files directly in the messaging service without actually owning any of the products.

Nearly 35,000 people watched Facebook's live ustream feed of the announcement.

Reach executive producer Paresh Dave here. Follow him on Twitter: @peard33.