If I'm not mistaken, you buy ads on a cost-per-click or a cost-per-thousand-views basis. In other words, you only pay when people see or click your ad. You also choose exactly what audience you'd like to target on FB -- very few companies are rich enough and broad enough that they want to target the entire FB userbase.
Long-story short, Facebook's total monthly active user count is largely irrelevant to individuals purchasing ads.
You don't need to have more views or clicks. If you claim you have 2 or 3x the number of monthlies, you charge more per click or view. You'll attract more advertisers to your site which further drives up the cost of each advertisement.
In this way, inflating monthlies has a direct effect on ad revenue.
This doesn't even make sense. If you are charging per click (or view), you don't charge more because of number of visitors, you make more because Revenue=CPCCTRViews (and Views is getting larger).
Yes they do. The post you are replying to even explained that. If you tell the world "we have a billion users", more companies will decide to purchase advertising on your site than if you tell the truth and say "we have 400 million users". More advertisers competing for the same number of page views means they drive the price per view up, just as any other increase in demand without an increase in supply will drive up prices.
Long-story short, Facebook's total monthly active user count is largely irrelevant to individuals purchasing ads.