There is a recent Hacker New thread about Twitter reading the installed apps on your phone to send that information back to Twitter and Twitter in turn sells it to whoever will pay money for it.
I have often lamented about ad-based business models on TechCraft. They are in direct conflict with users interests. It is not so much that the app is reading your installed apps, that is unsurprising. The problem is that it is done by default and you have to opt-out of it. Therefore I have un-installed Twitter from my iPhone.
I have heard the argument that most people “don’t care” about this or they are “OK” with it. I think that that is a fundamentally lazy defense and demonstrates a lack of understanding of the userbase. Ad-based models are a race to the bottom, no-one wakes up thinking they would love to see more tailored ads. This model degrades the product and the user experience and ultimately shareholder value.
Twitter FAQ here: https://support.twitter.com/articles/20172069