usecases = ["group events", "messaging", "staying in touch with people", "networking", "checking in once a month", &c()];
singleUsecase = pickWhatevs(usecases);
otherStuff = pickNotWhatevs(singleUsecase, usecases);
responseStr = "Well, I only use it for " + singleCase + ". I rarely use it for " + otherStuff + "."
As a former employee you might have more insight than I do, but how exactly has a glorified message board which doesn't charge its users become one of the richest companies in the world?
Sure they're a bit interested in the fruit growing hobby and that network, but they're also very interested in the ways you "hardly use the site" and they're even more interested in the things your doing when you don't even know they're still watching you. Even more interesting, you're actions on and off their platform can be correlated with heavy users of the site to infer traits they'll assign to you and sell on, true or not.
At the end of the day, the way the species is evolving with the internet, giving someone permission to follow you around the web and collect that data on you is the same as giving them insights into your inner psyche, the majority of what composes your reality, and the things you value most in your life, among other things.
These are things we should have a right to offer explicitly, not implicitly surrender.
[edits to elaborate on a couple points and add helpers to the why I use FB excuse]