Starting this Tuesday, MySpace is giving all developers the chance to start building their own applications for the site using its developer platform. Then, in a month, the company will let users start adding any of these creations.

MySpace is the largest social network in the world, with more than 110 million unique visitors a month, so the platform could quickly rival Facebook’s, which itself has spawned thousands of applications that are used by millions.

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Because of MySpace’s heft, a few companies will likely see the vast majority of their users embrace these MySpace applications — especially since Myspace applications will soon begin working on other Open Social member social networks, like Google’s Orkut.

Companies like Slide, RockYou and others that have grown large widget empires both on MySpace and most other social networks, and have also gained millions of users on Facebook. Their MySpace widgets, if nothing else, are massive advertising vehicles for their new apps. Their Facebook applications are where they learned how to use developer platforms to virally gain users.

In fact, these companies — Slide, RockYou — need to win. They need to reach as many users as possible, because they’ve taken millions in venture funding, they’re still trying to figure out ways to monetize, and they’re relying on reaching as many social network users as possible to scale whatever business model ends up being a hit. MySpace, despite Open Social’s inclusion of other social networks, is the last big social network target.

The Facebook advantage:

The official line is that tomorrow any developer will be able to access MySpace’s specialized platform code documentation, sample application code, a developer sandbox for creating and testing applications, forums, and blogs from the platform development team, as Adam Ostrow notes in his early look at the platform.

MySpace appears to leverage the many third-parties that already own widgets on millions of its users’ profile pages. It has added a feature, code called “action scripts,” in its platform that will let existing widgets use the platform to let the widget’s users easily invite friends, or add user data such as photos.

Widgets are snippets of code that are embedded in a web page and, besides action scripts, don’t normally have any way of connecting to data contained in the page they’re located on. Applications are more complex because they use social data, such as lists of users friends, to spread virally.

Take the most simple instance of an application platform at work. You can invite 20 Facebook friends to an application, using Facebook’s application programming interface provided in its platform to access your list of Facebook friends. A widget on MySpace has until now had no way to know who your friends are, nor a way to invite them to add the widget using Myspace’s interface.

The reality is that the biggest winners among Facebook application developers, such as Slide, RockYou and others, know how to use these viral channels better than a MySpace widget-maker that has never built an application. These two companies, and others that already have millions using their Myspace widgets, will be able to experiment with application-izing their MySpace widgets even as they build MySpace versions of their successful Facebook applications, using their knowledge of what works for growing applications.

Slide has just raised $50 million on a $550 million valuation, with the intent to reach the most users in the world (it already reaches more than 150 million, according to Comscore). It is developing ways for advertisers to reach users. Example: Branded items, like an image of a movie star, that shows up within a user’s widget or application.

RockYou has also raised millions in venture funding. It sees a big opportunity in selling advertising on its applications that lead users to add other applications.

Slide, Rockyou and other widget-makers have recently gained the attention of advertising agencies on Madison Avenue, that are experimenting with ways they can use these social networks to reach massive numbers of users. If these widget (and application) makers can figure out how to pair users and advertisers in a way that users can deal with, and that pays, their scale means they will make lots of money.