The September issue of Business 2.0 magazine has a good article on the transformation happening at Facebook (the No. 2 social network with 32 million users), since it opened its platform to hundreds of developers a few months ago, providing access to the site's audience and the programming tools needed to draw them in.

It is transforming the site into a new kind of software platform for "widgets" - small applications that allow users to do everything from scrawling graffiti and sending virtual cocktails to buying music, brokering loans, and joining causes - usually without leaving their Facebook homepage. In just 10 weeks, more than 2,500 new widgets were created, triggering 140 million downloads. IPG has formed a strategic partnership by acquiring a stake in Facebook; venture capitalists are setting aside millions of dollars to develop even more powerful widgets.

To help find the most popular new widgets, Appaholic.com tracks the most popular and the fastest growing apps, which can double in popularity overnight as they are shared by friends through the network. Its viral dashboard allows you track, sort, and graph the popularity of every widget on Facebook. As of today, the 5 most popular widgets are Top Friends, My Questions, Video, Graffiti, and iLike.

Widgets offering the best sponsorship and branding opportunities are Likeness (3 million users, offers quizzes that generate top-10 lists), FoodFight (2 million users, throw virtual food at your friends), and HotLists (1.6 million users, post brand logos to help define your persona).