What would it mean to have a stripped down social network that was also secure? It would look like Tuenti, the social network out of Spain. To give an example of how seriously they take security, the Tuenti API is closed because "of our efforts to be the most private, secure social network on the market..." But if you play nice, they can make exceptions for commercial apps and games they have tested.
Yahoo is weighing its options with its blog-based social network tool MyBlogLog and has not ruled out closing down the site--and its API (our MyBlogLog API profile). When the MyBlogLog API launched, it looked to have a promising future, though this was before Facebook and Google released similar features.