a) Vaguely agree that Facebook is awful, and removing the app is a good idea,

b) Start explaining how a detailed log of every app user's location, time spent reading each story, typing speed, searches, lists of other apps used (or in use), full lists of contacts, calendar events, text messages, phone call history, photographs, nearby WiFi networks and battery statistics can be combined with website-based Facebook use and tracking cookies on other sites, allowing Facebook to create a detailed profile of each user, and that's why they showed the creepily-relevant advert.

I would usually choose (a), unless the discussion is specifically about data privacy. (b) is not literally listening, but it's close enough for a casual discussion.