That works with e-commerce, but not necessarily with advertising. If you were going to browse Facebook during lunch in the office (and be shown ads), but Facebook is down, then you won't necessarily just "browse Facebook later in the day instead." You weren't on Facebook to catch up; you were on Facebook because you had nothing better to do.
This is definitely true, I expect that users will spend a bit less time on Facebook today.
However, Facebook doesn't have a 100% fill rate of high value ads, so during the rest of the day they'll be showing somewhat more expensive ads to users, earning a higher average revenue. That will make up for some of the loss, despite lower usage.
