As of Sunday evening, Facebook users can try out a new profile design. While part of the redesign seems aimed at cleaning up the site’s interface, the changes also affect the third-party applications that run on Facebook, undercutting some of the techniques that application developers use to expand their audiences.

Facebook opened up to third-party developers just over a year ago, with the launch of Facebook Platform in May 2007. Developers host their applications on their own servers but make them available through Facebook’s interface. Facebook users can browse through lists of applications and add any that they find interesting to their profile pages. Users can grant applications permission to access some of the information in their profiles, to publish notices that appear on the profile pages, and to send notices to their friends. Developers raced to get access to Facebook’s users, who the site claims now number more than 80 million.

However, the initial gold rush had its excesses. A user who installs a Facebook application is given the option of recommending it to his or her friends, but some developers made it difficult to skip that step. Facebook responded by forbidding applications from preselecting long lists of users’ friends to receive notifications. The company also suspended a few high-profile applications that it said weren’t respecting Facebook’s privacy policy. The profile redesign continues the process of reining in third-party applications, while adding features that the company says will make for a better user experience. “Third-party applications are more tightly integrated into Facebook in order to make using applications simpler and more seamless,” the company said on a Facebook page dedicated to the redesign. “Users will have the option to interact with an application before adding it, grant it access to their information, and decide where they want it placed, if at all, on their profiles.”

Specifically, the profile redesign splits the existing profile into tabs, emphasizing the wall, where a user’s friends leave him or her short, semipublic messages, and the news feed, which describes the user’s activities on the site. Most important for third-party developers, it removes most of the applications that the user has installed from the main page of the profile, relegating them to a “boxes” tab instead. To compensate for the applications’ reduced visibility, Facebook allows users to add tabs for their favorite applications, presenting the possibility that developers could design entire sections of a user’s profile.

Moving applications off the profile’s main page could have a big effect. Although users can search for interesting applications in Facebook’s application catalogue, they can also install them when they notice them on their friends’ profiles. “A very large portion–upwards of 20 percent–of application installations now come from the profile,” says Nick O’Neill, owner of Capital Interactive, a company that provides social-media consulting and Facebook-application development services. (O’Neill also runs the blog All Facebook.) It remains to be seen, he says, how the changes will affect that number.

The redesign is in line with some of Facebook’s recent policy changes. In a June post on its developers’ blog, the company writes, “Users must not be surprised by the outcome of an action they take.” The redesign includes controls that give users the ability to finely tune what applications can do to their profiles.

“The profile redesign is going to change the application landscape entirely,” says Jia Shen, chief technology officer and cofounder of application developer RockYou. “What I always say is that developing on a platform, especially Facebook or a social-networking platform, is like playing a game where the rules change every couple of months. This is a big rules change for the ecosystem.”

Shen notes that, in addition to adding the boxes and favorite-applications tabs, Facebook is changing the rules for posting notifications on users’ profiles. Facebook will also ask users to opt in each time an application requests access to additional profile information, rather than simply granting the application blanket access during installation. Shen expects the change to make users a bit more squeamish about giving applications access, but he hopes to allay their fears with graphics that help them understand what they’re allowing RockYou’s applications to do. Shen says that RockYou has plans to promote the use of application tabs in hopes of getting a jump on its competitors. He notes that the first applications users add to the tabbed section will have an advantage, since the new design makes the first three applications the most visible.

Justin Smith, a product manager at application developer Watercooler, who also analyzes the development of Facebook Platform on Inside Facebook, says that the changes could help intrepid developers shake up the status quo. He notes that some developers have found it hard to attract new users since the initial rush. “I think that any time there’s a structural change in the ecosystem, there’s new opportunity, because not every old developer will be quick to adapt to the changes,” Smith says. New applications that make good use of features such as the application tabs might have a chance of upsetting the current leaders, he points out.

But Capital Interactive’s O’Neill thinks that developers won’t build with Facebook Platform forever. “The real question is, what are people doing when they’re on Facebook?” he says. “They’re either looking at profiles or managing their contacts. The other things people do are really not limited to Facebook. You can already go play games anywhere else on the Web.” He thinks that tools such as Facebook Connect, which allows developers to bring features of Facebook into their external sites, could ultimately become more attractive to third-party developers and reduce use of Platform.