The Twitter Streaming API has been successfully used by Yahoo!, Google, and Microsoft, who have been given access to the full feed of all public tweets. Twitter has now enabled this "firehouse" of data for a number of other companies.
Microsoft has announced its new Facebook SDK, which allows developers using Microsoft technologies to quickly create web client (via Silverlight), web server (via ASP.NET) and desktop (via WinForms and WPF) applications that harness the power of the Facebook API.
Involver launched its Engagement Optimization API last week (read the official release here). Partners are utilizing the API to "optimize their ad spend on social in the way they've been doing across the web for a long time." Although advertising across social networks has been deployed for quite some time, advertisers have had limited tools to track engagement. Facebook Insights is probably most commonly used to analyze social data. When users have required deeper analysis, the advertisers were forced to use highly custom integrations.