PC World has learned Facebook will announce Monday that it is building a full-fledged email system into its 500 million member-strong social network, according to a source within Facebook. The official announcment of the new email offering will come Monday at a planned Facebook press conference at 10 a.m. PT.

Now users won't have to leave Facebook to send e-mail to their friends, but rather click into a new email link in Facebook to compose and send messages. The Web-based email offering has been created from the ground up, according to a Facebook employee familiar with Monday's announcement, and is not just be an improvement of Facebook's existing limited Messaging system. The email client will allow you to swap e-mail with anyone within Facebook or outside Facebook that has an email address.

Watch Out Google

The move makes sense -- it's simply the latest volley in Facebook's war with Google to be the place where people spend most of their time on the Internet. Google currently owns this honor, but Facebook has proven that it's addictive service can hold people's attention for long periods of time. In terms of Internet advertising and marketing, this is extremely valuable.

Facebook

Facebook is clearly trying to match all the services Google provides around its core search capability.  Beyond its messaging capabilities, you can also already share links, photos, and video clips as well. There's an instant messaging chat tool. A partnership with Skype allows you to make voice calls from the platform; Google only recently added this capability to Gmail. E-mail is the piece that has been obviously missing, and now it is here.

We will be getting a look at the actual Facebook e-mail client later this morning, and will report back with more details. It is, however, expected that 'vanity' Facebook URLs, like www.facebook.com/marksullivan will turn into email addresses, like [email protected].

There is certain to be more privacy discussion around this move, as Facebook begins hosting our e-mail data at its servers. What new information from our e-mails will Facebook collect and add to our social graph? But it's the same old bargain: are you willing to give up a little more of your privacy in exchange for a free and convenient new service?

More details to come during the Facebook announcement here in San Francisco.