How solid are those numbers? 25% sounds like a lot to me for your 'average user' kind of people, who make up the vast bulk of people on the internet. The curve also looks a bit optimistic.
My guess is that they have a script that observes ad placements and counts "non-impressions".
Counting impressions at the site level is not the same as counting them at the ad server level. There are generally multiple ad servers daisy-chained into the ad tag, creating a lot of latency. It is very typical to lose 10% or more ad inventory simply because the ad can't load in time.
If pagefair can't account for that, then the number is highly inflated.
Not solid at all, at least according to the traffic I get. In verticals like HN it is quite likely to be high, but in the real world it's not. If anything I would wager that AdBlock usage is declining because of the dramatic increase of mobile traffic.