Every day, Facebook serves up many, many millions of ad impressions. But the world’s largest social network wants its marketing partners to know it doesn’t even start counting those ads — or charge for them — until an actual human being sees them.

Although this has become an industry standard, and has long been Facebook’s practice, the company said this morning that it has never before publicly discussed its philosophy towards counting ad impressions.

Along with its recent announcements that it has launched a new ad unit known as Product ads, which allow marketers to highlight exactly which products they want to promote, and its new system for measuring ads’ relevance, it’s been a big week for developments on the business side of a company with a current $211.6 billion market cap. Much of its recent activity has been geared toward boosting its value to marketers, an approach which in turn bolsters Facebook’s own bottom line.

“At Facebook, we have always been focused on delivering value for advertisers,” the company said. “We agree that viewed impressions are a better standard of measure for marketers, which is why we use viewed impressions to measure ad delivery across desktop and mobile. We don’t charge for impressions that aren’t viewed. The reason is simple: impressions that aren’t viewed create no value for advertisers.”

While the company could easily count — and charge — for served impressions, it knows that in the long run, everyone wins if it only tallies them when they’re seen. Just because the ad shows up in a user’s Facebook feed doesn’t mean that person actually saw it. “For instance, the ad could appear far down at the bottom of a web page (below the fold),” the company said. “Or a person could visit a site and then leave before the ad has fully rendered.”

In the end, Facebook said, only counting ads that have been seen makes sense in every way.

That said, Facebook also noted that it counts the impression the moment any part of it is visible to a user, regardless of whether or not that person is in any way aware it is there. As any Facebook user knows, that means the company is tallying countless ads that are more or less ignored, as well as the ones users actually notice. As long as it’s on the screen and can technically be spotted, it counts.

Still, Facebook argued that this is at least a better and more meaningful system than counting every ad served. “Our research shows that value is created as soon as ads are seen by people, even if only a portion of the ad was seen for a brief period of time,” the company said. “Conversely, an ad that is served but not seen delivers no value for an advertiser.”

The company also explained that it uses the same measurement standards across every platform and ad type, meaning marketers can depend on a consistent system regardless of where their messages show up. Plus, the viewed-ads approach ensures fair pricing, Facebook said.

In a draft blog post on its ads philosophy, Facebook noted that a wide range of advertising partners, from Wendy’s to the ZenithOptiMedia Group, have lauded its approach, which, it said, measures up to a 2014 standard set by the Media Rating Council to count ads only when an “opportunity to see” them is established.

Facebook did not explain why it chose now to elucidate its ad measurement methods.