What you are getting at however is very important to ad-driven internet business models. You make the empathetical deduction that if you are there for one purpose, you are impervious to advertising.
Now, if you are still with me after that statement, then there are larger impications of online advertising than are actually being measured. For instance, if the intent of the view mattered, would it be worth more? I think Google has answered that question rather clearly.
Measuring "why someone is visiting a URL" (and thus generating an ad-view) is a larger technical problem than could be accomplished with some fancy algorithms. That is, unless the service knew all of your browsing history...
In any case, you raise a good point about the intent of the page-view and how that should be "credited" as advertising. To do this however is to challenge existing 'fundamental' theories.
Also, there is "brand marketing" and "performance marketing." I'm not sure they have classic definitions, but generally brand marketing is based on "mindshare" and tough to quantify. This is basically any TV commercial -- Budweiser doesn't expect you to jump off your couch the second you see a Bud Light commercial, but maybe a week later when you're in the supermarket, you spot that new brand of beer and recall the commercial with Jay-Z rapping and grinding with some models, and think, "eh, maybe I'll try Bud Light Platinum, I will purchase this and consume it later because I like the idea of rapping with models," even at a subconscious level.
Performance marketing is all about ROI. You spend $100 marketing dollars, and you want to make at least $X back. Typically $X = 100 in this example, although in some cases you may want to adjust profit margins or even run at negative profit (for example, if you want to maximize revenue even at an unprofitable rate). Almost any advertiser on the internet has a performance marketing mentality, mainly because it's much easier to actually measure performance. This is why advertisers like the OP are frustrated with Facebook, because paying for bot traffic makes it extremely hard to hit their performance marketing goals.
You are right though, in that this is a huge gray area. To give two extremes, let's say you run an ecommerce site but you'd also like to monetize with ads. So you reach a deal with a shopping comparison site to display their products on your pages in your footer. If you buy AdWords to your site, and those visitors from AdWords scroll down and click on the shopping comparison ads, they are probably fine with paying you for that traffic, although you are probably paying something like $2/click. If you make a page with JUST the shopping comparison ads, and buy traffic from someone who basically infects computers with trojans and overwrites their Google search results to be your site (say at $0.01 per click), then you'll get clicks on the shopping comparison ads and make lots of money, except the shopping comparison site will realize these clicks don't lead to any actual sales for their merchants (since this is extremely poor quality traffic since all the users that end up there have basically no intent) and will kick you off. Somewhere in between adveritsing for "first page Google AdWords" traffic and "trojan/botnet" traffic is basically every other form of internet advertising, and how gray that area is, is typically completely defined by your business partners.
Advertising and marketing are generally very intentionally tailored. You're not trying to reach any human, you're trying to reach a human in your target demographic.
Skewing results (and wasting resources) by paying people who are entirely outside your marketing effort to click through links (or take other similar resource-costing / data-skewing actions) is not the desired result.
See, the essence here is not to come up with a great algorithm. This is not a programming contest. But to create a system where everyone (publisher, advertiser, ad-network and customer) wins.
In this very early stage, the success of Facebook's Ad network should be measured only by one metric - ROI of Advertisers. And that number has to be consistently better than Google's.
Why's that? This isn't a zero sum game. You don't advertise only on Google or only on Facebook; as much as your budget allows, you advertise on every channel you can achieve a positive ROI on.
For any advertiser looking for a direct response, like a signup or purchase, I doubt anything Facebook can do would make the ROI better than advertising on search. No amount of demographic and interest targeting will make advertising to people as effective as advertising to intent. You can pay Google to send you people while they're in the process of looking for your product; Facebook doesn't do that.
A combination of tech expertise and marketing savvy are needed to reduce this problem. More analytic tools are available for Google Adwords than are available for Facebook ads.