Facebook routinely tracks users, non-users and logged-out users outside its platform through Facebook Business Tools. App developers share data with Facebook through the Facebook Software Development Kit (SDK), a set of software development tools that help developers build apps for a specific operating system.

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•  Typically, the data that is automatically transmitted first is events data that communicates to Facebook that the Facebook SDK has been initialized by transmitting data such as “App installed” and “SDK Initialized”. This data reveals the fact that a user is using a specific app, every single time that user opens an app.
• • In our analysis, apps that automatically transmit data to Facebook share this data together with a unique identifier, the Google advertising ID (AAID). The primary purpose of advertising IDs, such as the Google advertising ID (or Apple’s equivalent, the IDFA) is to allow advertisers to link data about user behavior from different apps and web browsing into a comprehensive profile. If combined, data from different apps can paint a fine-grained and intimate picture of people’s activities, interests, behaviors and routines, some of which can reveal special category data, including information about people’s health or religion. For example, an individual who has installed the following apps that we have tested, “Qibla Connect” (a Muslim prayer app), “Period Tracker Clue” (a period tracker), “Indeed” (a job search app), “My Talking Tom” (a children’s’ app), could be potentially profiled as likely female, likely Muslim, likely job seeker, likely parent.

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Some apps, like KAYAK share everything with Facebook:

• “KAYAK”, which sends detailed information about people’s flight searches to Facebook, including: departure city, departure airport, departure date, arrival city, arrival airport, arrival date, number of tickets (including number of children), class of tickets (economy, business or first class)

Source: How Apps on Android Share Data with Facebook – Report | Privacy International

About 90% of Android apps have Google embedded tracks and about 42% of Android apps have embedded Facebook trackers.

Many of these apps merge data between devices, including Android devices, televisions, and personal computers. They also track the web sites you visit and the links you click on.