The Service API classes provide a programmatic interface for interacting with many popular services. BigTree's Developer area guides you through setting up the API keys / clients needed for interfacing with the services but stops there as far as integration goes. Once a Service API is setup in Developer the related class will be functional.
Many Service API classes have related classes for object-oriented interaction. For instance, the Twitter class can return BigTreeTwitterTweet objects that have methods such as favorite and retweet. This object-oriented approach is forward looking and is most likely to be a model for BigTree 5's data interactions in the future.
BigTree provides a fairly robust Twitter API class complete with posting tweets, uploading pictures, pulling timelines, managing your followers, blocking users, favoriting tweets, and searching Twitter. See the BigTreeTwitterAPI class documentation for a full list of methods.
BigTree's Instagram API provides an almost complete implementation of the Instagram API with the large exception of posting content which is reserved for pre-authorized developers. See the BigTreeInstagramAPI class documentation for a full list of methods.
BigTree's Google+ API provides a read-only API implementation for common usage scenarios such as reading a user's activity, getting a list of friends, getting comments on activity, and searching the public Google+ feed for activity. See the BigTreeGooglePlusAPI class documentation for a full list of methods.
BigTree's YouTube API provides a robust API implementation that allows you to do pretty much anything but upload a video to YouTube. This includes searching for videos, getting user information, getting a user's videos, managing playlists, posting bulletins, rating videos, and more. See the BigTreeYouTubeAPI class documentation for a full list of methods.
BigTree's Flickr API is fairly extensive and allows you to upload photos, manage photos, search for photos, manage your friends, get your photo stream, and more. See the BigTreeFlickrAPI class documentation for a full list of methods.
BigTree's Disqus API allows you to manage your discussions programmatically. It does not implement the full Disqus API, but implements a lot of common use cases such as getting comments waiting approval and managing them. Most of the Disqus API is implemented through its child classes, so be sure to read the main BigTreeDisqusAPI class and its related classes as well.
BigTree's Salesforce API tries to provide you with some nice hooks to deal with what is generally a very abstract API. Because Salesforce data is so custom, there's not much you can do with a nice API interface. What BigTree does provide is a way to lookup the available "objects" in Salesforce (equivalent to tables in MySQL) and add/edit/delete/query the records in those objects through methods similar to BigTreeModule's methods. You'll generally want to look into the BigTreeSalesforceAPI and BigTreeSalesforceObject classes to see what BigTree's APIs can do for you.