The Yammer SDK for Windows Phone is available to interact via mobile in enterprises, startups, and small businesses. This SDK permits users to login with the IE browser, access an authToken, and make calls to the Yammer API.
At the bottom of many blog posts (including this one) and website entries you're likely to find a widget that allows sharing to social sites or emailing to a friend. ShareThis competes with other systems like AddThis and SocialTwist to give publishers an easy-to-install sharing method.
Why did Pinterest remove its API documentation earlier this year? Well, apparently there is no rush to release an already developed API to the public yet. In fact, it may not be released for a while as Pinterest wants to avoid re-tweeting the #mistakes Twitter made in its infancy.