It's used for balancing payments/debts between individuals. I don't consider "A paid B for brunch yesterday" or "X paid Y for Rent+cable" in any way to be "exposing finances". It's just another random (albeit unnecessary) social network, one that takes literally no effort to use or disable.
Shocking, young people don't care if others know that they went out to dinner with some friends. It's amazing to me that this level of exposure seems "nuts" to you all and leaves you "speechless". Like, how paranoid and antisocial are you all? Who cares about this stuff? Or is this all just some crazy slippery-slope thing going on in your heads?