DAU/MAU ratio tells a lot more. It's often called "stickiness." It's a good metric of who continues to come back to your platform.

Those numbers would still be juiced in the same way with artificial traffic. Revenue / user or revenue / request is less easily gamed as the more artificial traffic you add the larger that drops. Of course that's a lagging indicator but at least that forces Reddit into a position where they'd have to be defrauding their advertisers (or overcharging them on impressions which they'll push back on).

I'm not sure. But the answer should not be, for example, Instagram doing literally nothing about the sea of bots.