Important to remember that advertising revenue _always_ peaks in Q4, and that was an election year too, so even if they're down YoY this Q4 I wouldn't worry too much about it.
The scale is nowhere close. Maybe if you include black money, India might be a contender, but how much of that is spent online?
Not much. And besides, I don't think you could spend it online even if you wanted to. Unless Facebook is accepting cash payments for ads in India ;-).
You pay influencers/fake news outlets/biased media sources.
Though, not much is probably spent in India this way, but it's definitely there