Earlier this week, Twitter has announced its plans to start collecting information about the mobile apps that its users have on their phones. The Twitter App Graph, as the feature is called, is a step to better improve the company’s advertising business, which is its main revenue source.

According to Re/Code, the move will also help “build a more personal Twitter experience” for the users. Besides using the data for better targeted ads, Twitter will also use it to provide better “who to follow” suggestions as well as to surface more relevant tweets, accounts and content in users’ timelines.

Through the App Graph, Twitter will collect the list of apps downloaded by each user, although they will not be getting information about the activities within the apps. For example, Twitter may know that a user has downloaded Spotify, but it can’t know which tracks the person is listening to.

Now, how will Twitter do that? It turns out the company will tap into a hidden feature in the iOS and Android APIs, a sort of spyware that lets developers scan devices for installed apps. You could imagine thousands of developers cursing Twitter now for spilling the beans – and they are actually.

But then, Twitter isn’t the first and only company that’s using this strategy to improve their advertising efforts. Facebook has a similar app tracking tool, which collects data on the apps users have downloaded. The only difference is that Facebook only does this for apps that use its software developer kit.

Twitter App Graph is Opt Out

Twitter’s app tracking feature is reportedly opt out, which means that the social network will automatically collect information from users unless they explicitly say otherwise. The company says this already excludes users who have previously opted out of ad tracking in their Android or iOS apps. If a user opts out, any previously collected data from his phone will be deleted from Twitter’s servers.

Users will receive an in-app notification that says “To help tailor your experience, Twitter uses the apps on your device.” Below the message is a link that will redirect them to their settings.

Re/Code shared a screenshot of it:

The app tracking feature has been rolled out on iOS devices on November 26th and will roll out for Android users in the following week or so.