In a way, they _do_ have a limited-time discount. Facebook's ad prices are set by an auction. If there are less advertisers competing during the boycott, the prices will be lower.
Ad rates are down, and I know several publishers, whose ads are monetizing well, are using this opportunity to increase FB spending . Like, buying hand over fist
I can see the rationale for this. Companies that publicly boycott Facebook harm Facebook's reputation (which in turn hurts Facebook's bottom line). These companies now pose a higher risk (given past behaviour) of publicly boycotting Facebook in future. So Facebook should insure against this risk by charging a 'boycott risk premium' (through higher advertising rates) for companies that are a proven public boycott risk.
