Custom facebook backgrounds do not exist. Since its inception in 2004 as an online community for Harvard students, Facebook has set itself apart from other social networking sites such as MySpace and Xanga by insisting that its users abide by a strict style guide, only using text and Facebook-defined image guidelines to personalize their pages.
This might seem to be extraordinarily strict for a site supposedly designed to be an outlet for communication and self-expression, but it has worked out. In spite of being unable to customize Facebook backgrounds and other aspects of their sites, more users have chosen Facebook than MySpace or any other social networking site. Several buyouts of Facebook have been attempted, going as high as $750 million, but the owners have held fast, keeping Facebook independent (though they exclusively sell adspace from clients of Microsoft's ad pool, which would appear to be a somewhat exclusive sponsorship).
One reason that Facebook backgrounds are not customizable is security. Usually the methods used to control layouts and imagery on sites such as MySpace is CSS. Cascading style sheets are a method that web designers use to separate the appearance of a web page from the content, enabling sites to be shown more clearly on multiple platforms such as phones, computers, or webTV. However, as with any code being executed by a web browser, it creates vulnerabilities for malicious coders. Since Facebook has had a history of breaches in security and user information, eliminating that particular danger was a prudent decision on their part.
Another aspect of limiting background images is the elimination of any issues of impropriety or violation of community standards. Facebook was originally a very exclusive application - only available to students of Harvard, then of all Ivy League students, then to corporations such as Apple Computer, but now has extended open membership to everyone over the age of 13. This means that they have to be careful about what types of images are shown to those under the age of 18. While there are certainly ways around that issue - Flickr and Livejournal have "mature content" filters, for example - the easiest way to avoid the possibility is to eliminate the option.
Alternatives to Facebook Backgrounds
This is not to say that there are not a lot of great ways to personalize a Facebook page. One of the advantages of Facebook is the availability of externally developed applications for members to enjoy and send to others. For example, the "Flair" application shows a graphic of a little corkboard and then presents the user with a selection of "buttons" - images that can be put on the board with messages ranging from political slogans to silly sayings like "I'm not as random as you think I salad." These buttons can be arranged on the corkboard and sent as "gifts" to other Facebook users. It is also easy to created original "Flair" buttons as well.
Another graphic way to customize a Facebook page is with the Expressions application, which has a series of vertical banners with quotes, movie logos, and other pictures that users may enjoy. New images are created and added to the Expressions library, and the Facebook members can be notified about it and alternate what image is shown on their profile.
Having these applications and others displayed on the profile page of each user is one of the ways that Facebook allows a measure of personalization. The profile page itself is made up of many different elements, some of which are fixed in place but many of which can be moved. The two applications described above are able to be moved into many places on the profile page, and that constitutes a measure of control over the viewer's environment if not actually being Facebook backgrounds.
The top of the profile shows an individual's picture, and that is also completely customizable. In fact, Facebook differs from many other sites in that there is an unlimited number of photo uploads for users (though individual albums are limited to 60 pictures each).
In the end, the lesson taught by Facebook's popularity in spite of limited flexibility is that if there is a quality interface, the users will come, have fun, and make Facebook number one in social networking.