blog.pmarca.com: Open Social: a new universe of social applications all over the web:

In a nutshell, Open Social is an open web API that can be supported by two kinds of developers:

* “Containers” — social networking systems like Ning, Orkut, LinkedIn, Hi5, and Friendster, and…

* “Apps” — applications that want to be embedded within containers — for example, the kinds of applications built by iLike, Flixster, Rockyou, and Slide.

This is the exact same concept as the Facebook platform, with two huge differences:

* With the Facebook platform, only Facebook itself can be a “container” — “apps” can only run within Facebook itself. In contrast, with Open Social, any social network can be an Open Social container and allow Open Social apps to run within it.

* With the Facebook platform, app developers build to Facebook-proprietary languages and APIs such as FBML (Facebook Markup Language) and FQL (Facebook Query Language) — those languages and APIs don’t work anywhere other than Facebook — and then the apps can only run within Facebook. In contrast, with Open Social, app developers can build to standard HTML and Javascript, and their apps can then run in any Open Social container.