Facebook is the latest company to accede to the demands of advertisers and allow independent measurement firm Moat to determine if ads are actually being viewed. Up until now, Facebook and Google have both used internal measurement tools, which several advertisers — Unilever among them — have found to be an unsatisfactory solution. That’s the reason that Unilever, for example, has pulled back ads on Facebook and Google. Twitter has recently also permitted Moat to ascertain viewability figures.

The Wall Street Journal reports that Facebook is “also offering advertisers the option to pay for ads when the entire ad unit appears on the users screen.”

The Media Rating Council and Interactive Advertising Bureau’s guidelines, issued last year, define an online video ad as viewable if “at least 50 percent of the ad is visible on a user’s screen for at least two consecutive seconds.” That’s too permissive, say advertisers, who are lobbying for stricter measures.

The topic of ad viewability has heated up as advertisers are increasingly concerned that they’re paying for ads on Web pages that no one sees. Even Google has opined that, “only half the online video ads across the Web are actually viewable.”

In addition to Twitter’s June announcement allowing advertisers to use Moat, says WSJ, Google’s YouTube is “also close to offering advertisers the ability to use third-party verification,” an unverified statement that The Financial Times earlier reported.

“We’re committed to meeting all of our clients’ measurement needs through a combination of product innovation and industry partnerships,” said Google. “We have further efforts planned in this area and are taking our clients’ feedback into account as we continue to roll out new solutions.”