The built-in Facebook plugin SDK instance exposes the following APIs.
Getting the Facebook instance
When a project uses the Facebook plugin, the ObjectClass for it can be looked up using the runtime method GetSingleGlobalObjectClassByCtor(C3.Plugins.Facebook). From there the single-global instance can be obtained and then the SDK instance, which exposes the API below. Note that if the Facebook plugin is not present in the project then GetSingleGlobalObjectClassByCtor() will return null. The code sample below demonstrates how to access the Facebook plugin instance from another addon.
const facebookObjectClass = this._runtime.GetSingleGlobalObjectClassByCtor(C3.Plugins.Facebook);
if (facebookObjectClass)
{
const facebookSdkInst = facebookObjectClass.GetSingleGlobalInstance().GetSdkInstance();
// can now make calls, e.g.: facebookSdkInst.GetAccessToken()
}