Facebook, the social networking site that until now has been reserved mostly for college and high-school students, is preparing to throw open its gates to the great unwashed, the hoi polloi.

We’ve been briefed on the move. Facebook is saying in its briefing — sent earlier in the form of a statement to a few people, and followed up with a phone call — that the move will also keep Facebook’s network closed: “Closed Network Structure Remains Intact,” the statement reads.

Here’s how it will work. Under the plan, anyone can sign up, even with email addresses from Hotmail or Yahoo. But Facebook will give existing members the right to keep their profiles from being viewed by people they don’t want to give access to. Newcomers, or those without a college, high-school or work network email can only join as a member of a region. Regions were started last fall. So Liz Gannes for example, who is a member at Facebook, says she can see the profiles of the 17,412 people in Facebook’s San Francisco network. So that’s not very closed, and will be less so once the gates are thrown open to the multitudes. Existing members, though, have to opt in to be a member of the regional network. So there is still a separation of classes.

Now, these changes have been in the works for a while. Facebook had put out briefing feelers on this two weeks ago.

On the surface, this abrupt move toward openness flies in the face of the caution you’d think Facebook would have learned about privacy — caution it learned as a result of fierce protest from users last week in response to Facebook’s moves to expose updated profile information. However, it is clear that Facebook was headed in this direction, and wants to move forward, but is now testing the waters with users before doing so — and properly so!

So if you have any feedback, now is the time to submit comments, either here on this post, or at the blog of founder Mark Zuckerberg (pictured above).

The company said privacy controls will remain intact.

Here’s more from the statement:

Previously, a supported “.edu,” “.com,” “.org,” “.gov” or “.mil” email address was needed to join a collegiate, high school, or work-related network on Facebook, and only then could users join an optional regional network.

Expanding registration will make it possible for friends to join Facebook and interact with people within a particular region.

Unlike other social networking sites, Facebook has a closed, network-level privacy structure that enables users to establish individual privacy settings for each of their networks. For example, if a user joins Facebook at one regional or college network, the only people who can see their information are other people in that network and confirmed friends from other networks.

Users can only be a member of one regional network at a time and can only change regional networks a fixed number of times. Facebook’s registration and network authentication system establishes the accountability that leads to greater confidence and trust in a user’s online identity.

Within each network, users can choose who can find them in searches, who can see their contact information, who can view their profiles, and who is allowed to be their friend. Users can also set up a limited profile, which allows them to restrict access to certain parts of their profile to selected people. For example, a person can confirm an acquaintance as a friend, yet prevent him or her from seeing their photos or contact information.

In addition to network-specific privacy, users can submit security reports to the Facebook customer support team and Facebook deploys a variety of automated online monitoring tools to detect misuse of the service. The misuse of a Facebook account results in sanctions, up to and including termination of that user’s account.