[riboflavin]

I chose the word "addiction" pretty specifically - does it enrich the user's life? Uber does. (And you probably don't have a desire to use Uber for hours every day).

Social gaming (someone mentioned below) does, to a point, and then it's engineered to go way, way beyond that point.

It's complicated of course - and there's a spectrum - but I think a bad sign is if your architecture is intended to hack the reward systems of your customers.