mixi.jp, the most popular Japanese social networking service (SNS) with more than 17 million registered users has added support for mobile phones to Google’s OpenSocial platform. With this announcement, Google also introduced "mixi apps for mobile" platform which’s based on OpenSocial's RESTful API using 2-legged OAuth; with this OpenSocial applications can run anytime, anywhere! API (requires authentication using 2-legged OAuth). You can access all the core OpenSocial APIs, as well as photos and communities on mixi.
More info: mixi Developer Center