According to another commenter here, the research that spawned the WSJ article said Instagram made 41% feel better, 31% worse, and no impact for the other 28%.
I haven't found the original material to verify, but if this is accurate, seems like the net outcome argument works in Instagram's favor.
It doesn't necessarily work in Instagram's favor. If the 41% and 31% had a baseline of both feeling "ok" then you took 72% who had a baseline that is acceptable and polarized them into groups who felt better or worse. Is it more favorable to have more people who have a stable baseline or to have one group have a good baseline and another similar sized group with a bad baseline?
I haven't found the original material to verify, but if this is accurate, seems like the net outcome argument works in Instagram's favor.