What does the Facebook SDK primarily measure?

The Facebook SDK, or Software Development Kit, primarily measures actions taken on an application. This tool is instrumental for app developers as it facilitates the integration of Facebook services into their applications. By using the SDK, developers can track various metrics related to users' interactions within the app, such as logging in, in-app purchases, and custom events.

This tracking capability allows developers to gain insights into how users interact with their app, enabling them to make informed decisions about app features and marketing strategies. Understanding user actions within the app is essential for optimizing the user experience and improving retention rates.

Other options focus on different aspects of measurement not directly associated with the primary function of the Facebook SDK. For instance, user engagement on websites pertains more to web analytics, ad performance involves tracking metrics related to ads rather than actions within an app, and video completion rates typically relate to video content performance metrics rather than metrics specific to app interactions.