Why would you want to buy a “Facebook phone,” if—as is widely rumored—one is under development? 

Here’s why: if social networking is already the center of your online activity, a Facebook smart phone might be far easier to use. If a fraction of Facebook’s 800 million users were to make the switch, they would represent a powerful market force.

The phone project was revealed in reports last week that said Facebook had forged a partnership with handset maker HTC and was planning to use a version of the Android operating system, which will be tweaked to integrate Facebook deeply into its services and will support HTML5 as a platform for mobile games and apps. The reports said the phone could take 12 to 18 months to reach market. Facebook is saying nothing about the project; a spokesman said the company would not comment on “rumor and speculation.”

Already, Facebook’s are among the most popular apps on most smart phones. The company says that its apps on different platforms have 350 million active mobile users. The problem Facebook confronts is that its product is not very deeply “integrated,” in industry parlance, into the devices that people use socially every day to e-mail, send photos, and keep in touch with friends.  It’s just one of many apps people use.

For example, on an iPhone, if you open a Web page and click on the menu, you have the option of tweeting the link but not of sharing it on Facebook. That’s because Twitter got itself integrated, and Facebook, for whatever reason, did not. On some Android phones, Facebook is integrated in this way, but it could be even better integrated into the devices.

To use an iPhone to send a link to your Facebook friends, you need to take more steps to open and use the Facebook app. And much the same problem pertains to reporting your location, sending a photo, playing games, or engaging in any of a host of other activities.

But on a Facebook phone, such functions could be the default option. And people would find it easier to use Facebook itself—making Facebook an even more titanic Hoover of personal information than it already is.

Facebook could go even further by directing all communications—including voice and text messaging—through its platform. And it could use that same platform to deliver content, including music and video, to users.

“This could potentially shift the paradigm of what social networking and mobility can be and should be,” says Raymond Llamas, a senior research analyst for IDC’s mobile devices group. “Consider this: a smart phone that automatically checks you in on your location, finds your friends in the same area, uploads pictures of what you do to Facebook for all your other Facebook friends to see.”

The biggest challenge, Llamas says, would be convincing people to switch from existing phones, which do a pretty good job on many fronts, including providing a way to use Facebook via an app.

Then too, the company would have to expand the functionality of its phone beyond Facebook. “Here’s the whole crux of the situation for them,” says Mike Morgan, an analyst at ABI. “Is FB enough to make a device desirable? To this I would say no. The day of single-purpose devices has long since passed.”

Chetan Sharma, a mobile communications consultant, says that Facebook’s user base makes “entry into the [smart-phone] market compelling.” However, he adds that simply adapting the Android operating system may not be enough. “If they want be serious longer term, they might have to own a platform. They could also entirely focus on HTML5-based platforms and services and avoid the investment” of developing apps for a new operating system.

Al Hilwa, program director for IDC’s application development software group, says that in the long view, a giant like Facebook just needs to spread its reach, much the way Google branched out by launching the Android mobile platform and the Google+ social network. “If someone gets into your business, it’s almost incumbent on you to get into their business—otherwise, you get into a situation where they block you out. That’s not an immediate risk right now for Facebook, but that is one of the considerations. You want to try and own the whole data chain, end to end.”

Indeed, ABI’s Morgan says that a Facebook-centric smart phone is an obvious next step. “If Facebook wants a stronger mobile presence it needs to be deeply embedded so it can become part of the usage flow, so that more of what you do to collect and interact with people using your device ends up in the Facebook realm.  The more you use it, the more ‘sticky’ it is,” he says.

And the result will be a huge and sustained flow of information to Facebook, which helps the company. “In the end,” Morgan notes, “they are serving us ads.”