There's a change afoot for digital marketers — Facebook is pushing for a wholesale change in ad effectiveness measurement, moving from "impressions served" to "impressions viewed."
The idea is to more accurately track ads that were actually in the field-of-view for a reader; if an ad doesn't make it on-screen, then it shouldn't be considered as served.
The way it works now is that as long as an ad is loaded, it counts as an impression — even if it never makes it above the fold in the consumer's view.
For example, the user may leave the page before it fully loads, or they may never scroll down past the initial content above the fold. This means that advertisers are not only paying for invisible ads, but also deploying flawed statistics in analyzing the relative success of advertising campaigns.
In a blog post, Facebook explains how it wants to change this paradigm. With the backing of a juggernaut like Facebook, the hope is that the entire industry will shift its tracking mechanisms in favor of more accurate measurement. Facebook emphasizes this, saying:

Viewed impressions add an extra layer of analytical rigor, as well as common sense. They more accurately define delivery and help ensure that people have seen the ads they’re supposed to see.
The shift towards viewed impressions could affect your next social media marketing campaign by reducing the level of impressions that your advertising receives in a set period of time. So if you're comparing one campaign to another, there may very well be a measurable difference.
[caption id="attachment_150132" align="aligncenter" width="550"]
What counts as an impression?[/caption]
The other impact should hopefully be increased engagement.
Since the campaign analytics would only include ads that had the potential of being seen, the engagement should theoretically be a bit higher since a percentage of ads previously went unseen.
Viewed impressions will also deliver a more holistic view of how the copy is performing, as each ad impression was served above the fold with the potential to capture the user's attention.
Facebook also dropped a line in towards the end of the announcement under a header "How Facebook counts viewed impressions," which will have a guaranteed impact on any marketer that uses Facebook - either organic or paid (emphasis ours):

We measure an ad impression the moment an ad enters the screen of a desktop browser or mobile app. If an ad doesn’t enter the screen, we don’t count it as an ad impression. In the next few months, we will apply similar standards to organic content from businesses.
When it comes to organic content from businesses, only seeing served impressions will also give a better gauge of how the content is performing with followers. If no one actually saw a post, and yet Facebook is telling you that it did because an impression was served, that doesn't help at all to iteratively improve a business page's content.
In the big picture, this is a welcome evolution for social media and digital marketers. The industry is maturing, which means the focus then turns to creating compelling copy and content that engages readers.
Viewed impressions also ensure that the platforms continue to tweak the user experience to pull the reader down the page to be exposed to more ad units and organic content.
Facebook is working alongside stakeholders and the Media Ratings Council to deliver an industry-wide standard to define viewable impressions and create the accepted means to track these impressions.
NB: The eye image courtesy Shutterstock.