That means $875 could buy 1.75 million impressions.
Facebook might be more expensive, but even if it's $1 per thousand impressions, that's only $1,750.
I think the 1.75 million number in the article is incorrect.
Ultimately, it comes down to this: people aren't on Facebook to spend money. They're on there to see what they're friends are doing and communicate. If a friend posts a cool link in their feed, sure you'll get a few visitors following that link. But if the link is in an ad to the right, people just aren't interested.
This differs from certain aspects of the Google Display Network. For example, if you're on a Transformers fansite, you're probably there to geek out about anything Transformers. In that scenario, if you see a banner ad with a really cool Transformer's T-Shirt displayed on it, the ad almost becomes part of the content at that point. You're much more likely to click on it and actually be interested to see where it leads. (As opposed to accidental clicks, which makes up a huge percentage of cost).
[1] http://www.jperla.com/blog/post/facebook-is-a-ponzi-scheme
They are noting in the comments that 1.75 million ad impressions would probably only cost the advertiser a few hundred dollars, which would make it difficult for that to be the third largest advertiser considering advertising brought in a $1.86billion. I don't know anything about the cost of advertising, but do those numbers seem right?