The VKontakte .NET SDK by Artyom allows developers to integrate the VKontakte API into their C#/.NET applications. It can also be used with F#. This SDK was built under .NET Standard 1.1 and supports most popular platforms.
Viadeo, a social network founded in 2004 with 35 million professional members worldwide, is looking to take on LinkedIn as the preferred business social network in the United States -- and the company thinks that its Viadeo API will do it. Viadeo recently launched a publicly available version of its API that provides full access to the social network's users' data including career, news, groups, jobs, schools and a users status and messaging system.