Will Facebook be the service that makes the mobile Internet as widely-used as text messaging? The world’s leading social network has landed a potentially groundbreaking deal with some 50 mobile operators, mostly scattered across the developing world. The operators have agreed to provide access to a stripped-down version of Facebook (0.facebook.com) free of data traffic charges, meaning their customers will be able to send other Facebook users messages absolutely free, as well as posting status updates and writing on friends’ walls.

What is in it for the mobile operators? Well, free Facebook is effectively a big promotional tool for the mobile Internet and may persuade many of their customers to get online for the first time. Moreover, users will incur data charges as soon as they look at a photograph, video or follow a link to another site (they’ll be warned first). In the short-term, free Facebook may also give the pioneering operators a competitive edge – in most countries only one operator has signed up for the free Facebook service, although it is unclear whether any of these arrangements are exclusives.

500 million more Facebook users?

Although most of the 500 million people served by the 50 or so operators probably don’t have a PC or a Facebook account today, some will know someone who does and will sign up for a completely free Facebook mobile service. So, Facebook will almost certainly get many more users and the mobile operators should get some incremental data revenues, as people click on links.

To my mind, Facebook is getting the better deal. Facebook is not reimbursing mobile operators for the data traffic to the site and there are no financial terms to the partnerships, Henri Moissinac, head of Facebook’s mobile business, told Reuters. Moreover, the operators are running the risk that free Facebook will cannibalize some of their text messaging revenues. Although people are still likely to prefer SMS for private and urgent messages, there will be a lot of instances when communicating via Facebook will do fine. And there is one other major concern: Once you begin giving free access to even a small slice of the mobile Internet, you run the risk of resetting consumers’ expectations and notions of value.

Still, if the mobile operators market their data tariffs well and price them carefully and transparently, they could still come out on top. Ideally, when a free Facebook user clicks on a link to a photo, they should be offered the opportunity to buy access to 50 photos for, perhaps, the equivalent of $2 or $3.

The loss of text message revenues is likely to be greatest in developed countries, where prices are higher, making the business case for free Facebook more finely-balanced. Even so, a few developed world operators, such as BASE in Belgium, Vodafone and WIND in Greece, 3 in Hong Kong, SFR in France and 3 in the U.K., have signed up. (The American operators are noticeable by their absence). Others will surely follow and the rest will be watching this experiment closely.

But I doubt Facebook would call 0.facebook.com an experiment. Moissinac was at pains to stress to the Associated Press that it is designed to be a “permanent service” with all of the participating operators committed to making it free for more than 12 months.

And I suspect he’s right: Free Facebook is probably here to stay. It will likely usher in the mass-market mobile Internet and potentially even prompt another profound shift in the way we communicate.