A study by French research organisation Exodus Lab and Yale University's Privacy Lab found that more than three in four Android apps have a third-party tracker installed.
UPDATE 5/12/2017: Google has updated its Online Security Blog and said it will further enforce its Unwanted Software Policy to "further tamp down on unwanted and harmful mobile behaviours on Android".
This means that users will now be warned about apps collecting their personal data, so they won't be caught unaware if it's farmed. If the data collected is deemed "unrelated" to an app's functionality, then the app maker must disclose just what that data is being used for.
Under existing regulations, apps need to meet specific criteria outlined in Google’s blog announcement in 2017. Now, within 60 days, Google says, warnings could be issued on user devices about apps that collect data or websites leading to the said apps. This is good news for users who at the very least can then decide if they want to let app makers keep using their data for purposes that could include targetted advertising. Google recently banned advertisements from appearing on lockscreens so let's hope it keeps better tabs on information-hungry apps.
Probably the same for iOS
In their study, the labs searched for signatures of 25 trackers. These trackers are known to collect personal information about users to target them for advertisements and services.
The trackers were found on many apps, including popular ones such as Spotify, Uber, OKCupid and Tinder. While they may not necessarily be harmful by themselves, they do profit from your user information.
Another tracker, known as FidZup, can track user whereabouts via simple WiFi networks. This is more disturbing because it means that it reveals your location to advertising companies.
Does being on iOS offer more safety? While the study didn’t research iOS apps, it’s likely the situation is similar because many of the same companies responsible for Android apps also distribute apps and SDKs that run on iOS.
Basically if you’re seeing ads that seem way too targeted to be a coincidence, blame your apps. Perhaps be a little more sensitive to what you install and really read those T&Cs first. Yes, that download might be free cash-wise, but you might actually be paying a different price - your personal data.