Time spent on FB is time not spent on another site. And the higher-friction FB can make activities (to the pain point of driving away users), the better for FB.
There's a similar notion in other areas, and one of the most ridiculously malevolent examples I can think of is a brick-and-mortar sales promotion gimmick: a furniture store would run a midsummer sales event on a weekend where every family visiting the store would receive a free gallon of ice cream.
Because if you've just put a gallon of ice cream in your trunk, you can't visit another furniture store, you've got to head home to put it in the freezer. And time not spent in a competitor's furniture store is time not spent assessing and purchasing their products.
(That leverages the sunk cost fallacy as well, among others.)