Social-networking sites such as Facebook and others are deciding to open up their systems' application program interfaces to allow third-parties to develop software that interacts with those networks, according to BusinessWeek.

The APIs of these social-networking sites have been closed and closely held by the companies that developed them. Now, though, they're seeing the value in allowing others to create plug-in software that works seamlessly with their sites.

Facebook is reportedly about to open its APIs, thus far in a limited beta test. Other sites such as MySpace and Friendster are expected to do the same at some point in 2007, though they've announced nothing official about their plans.

Third-party applications can be an effective way to not only build community among members but also make more services available than could realistically be created by the company that owns the site.