But there’s a trade off. Fewer ads better placed means fewer people likely to block them and more people likely to actually look at them instead of skim past. It’s also likely that the people who were blocking ads are less likely to click on them and thus (for some advertisers) lower value impressions anyway.

Revenue is (simplistically) the product of impressions and value per impression. It’s not therefore immediately obvious that moves like this actually do increase revenue for them, especially over the long term since one potential side effect of doing this is giving more ammunition to the ‘delete Facebook’ crowd.