About six months ago, we reported that Facebook was working on a new product aimed squarely at the enterprise market under the working title, “FB@Work.” Now that product is officially coming to light: today the company is launching new iOS and Android apps called “Facebook At Work,” along with a version of Facebook at Work accessible via its main website, which will let businesses create their own social networks amongst their employees that are built to look and act like Facebook itself.

(Facebook At Work is now available for download on iOS, and we’ll update with a links to the Andriod version once it’s live, though both are usable via a limited pilot to start with. Check out Josh’s follow-up story for more screenshots, details on privacy, and analysis.)

Employers can create separate log-ins for employees to use with their Work accounts, or users can link these up with their other profiles to access everything in one place.

The product puts Facebook head-to-head with the likes of Microsoft’s Yammer, Slack, Convo, Socialcast, and a huge number of others who are trying to tackle the “enterprise social network” space. Even LinkedIn conveniently let drop last night that it too was looking at building a product for coworkers to communicate and share content (but not chat, as a LinkedIn spokesperson tells me). Not all of these have been a hit: Lars Rasmussen, the engineering director at Facebook who is heading up the project, had in his past once headed up one of the failed efforts at an enterprise social network, Google Wave.

Facebook is positioning today’s debut as a bold first step. “We’re putting the app into the app stores so that we can begin testing the product,” Rasmussen said in an interview.

In fact, Facebook has already been running tests of the service with “a very small set” of external businesses around the world, Rasmussen says; this is the next step in that process. The aim initially will be companies with 100 or more employees. (In fact, the existing Facebook Groups product is already used by smaller organisations.)

Because of the early nature of the product, there are a lot of questions in the air. The company has yet to work out, for example, how it might price the app, whether it will monetise the service through ads, or how third-party apps will work. For now, Facebook Platform has been disabled on the Work product, meaning no ads or apps.

That may not always be the case (“It could be paid,” he says).

This beta state of affairs is in some ways ironic. Rasmussen says that Facebook has effectively been working on Work for the last 10 years, because it is based on what Facebook’s own employees have been using to communicate with each other, pass on news, plan meetings and share documents. That long-time use and Facebook’s familiarity to all of us are part of what makes Facebook confident that it can carve a place for itself in a market that already is very crowded.

“Facebook at Work’s strength is that we’ve spent ten years and incorporated feedback from 1 billion active users,” he says. “All of that is embedded now in the same product but adapted for different use cases.”

And it’s actually used by staff. “When Mark [Zuckerberg, the CEO] makes an announcement he just posts it on Facebook at Work,” Rasmussen says.

In fact, Facebook’s own popularity could be Facebook at Work’s biggest advantage. A lot of efforts in offices to get employees to collaborate more with each other have been stymied because employees don’t want to use the software. It’s yet another new thing to learn and doesn’t feel essential.

A lot of messaging apps (Microsoft’s Yammer being one of the notably early movers) have tried to tap into “consumerization” — or getting enterprise apps to look and feel more like consumer apps — to encourage usage. In that vein, Facebook at Work, built essentially on Facebook itself, will be arguably the closest of all to an authentic “consumer” social experience.

Here’s a run-down of some of the key points about the service, as told to me by Rasmussen:

Pricing. As noted above, no firm details on this yet but consider that most of the other apps offer tiers of pricing. By making this free, Facebook could potentially drive a lot more users to its wider network.

The fact that Rasmussen would not rule out advertising as an option down the road to me suggests that Facebook could consider tiers of its own where some businesses may pay for the product and have it ad-free, while others might take it free and get ads. Again, most enterprise apps are based on a paid model so it’s probably more likely to remain the case here too. Plus this would give Facebook another revenue stream beyond ads and app-related payments.

How it will work. Facebook wouldn’t show me a demo ahead of the launch but this is how Rasmussen describes it: “When an employer adopts Facebook at Work, they can construct it with a set of new accounts. Users can then link their work and personal accounts together so that they are logged into both at the same time.”

This would work much like Groups and public profiles do today. On mobile, you would have two mobile apps running at the same time, he adds. “Even if the employee chooses to link there is no crossover. The content stays entirely within your personal or work Facebook.”

What’s not there/integrations. You can share documents today but for now there will be no in-app editing “currently.”

Again, that leaves this open as something that will come down the line. “The set of features are identitcal to personal Facebook, but just to get it out sooner we’ve disabled the Platform so the APIs that third parties work with are not there, but we are keen to turn it back on. Hopefully in the future other enterprise tools will integrate with Facebook at Work.”

Backstory on development. Back in June I’d deduced that Facebook at Work was connected with Rasmussen’s work in London, but what I found out from sources after that report was that this was more than casual: this was his baby.

Rasmussen is not shy to bring this up himself (which I have to admit is refreshing to hear, because people do like to posture a lot in tech, don’t they).

“I can say that the challenges of making work more efficient is something that has been on my mind for a long time, and I come to it with a lot of passion and the knowledge of a failure of doing this at a different company,” he says, referring of course to Google Wave.

“I thought that maybe Facebook’s experience was what was needed. When I worked on search here it was always at the back of my mind, so later I picked back up on that idea, joined in on the conversation.”

The caveat to all this, however, is one that Facebook will have to continue to grapple with, all the more so as it continues to grow.

Canvassing opinion on a Facebook at Work product, I heard not only once people shy away from the idea, concerned with the thought of Facebook “owning” your data and the potential lack of confidentiality resulting from it.

That can be frustrating when related to pictures of you too drunk when you were in college, or frankly scary looking when you were in high school, but potentially very costly and illegal if proprietary work information is involved.