Huge numbers of ads have disappeared from a tool created by Facebook to allow the public to know how they are being advertised to.
The major disappearance of data comes just two days before the UK heads to the polls in the general election.
The Facebook Ad Library is intended to collect all of the political ads that are being shared on the site. That in turn is meant to allow those ads to be scrutinised and seen by the public, since they may only otherwise show on a limited number of feeds.
The Ad Library has become a key tool for journalists and transparency campaigners, who can examine the claims being made on social media by political parties and check for misleading or false sponsored posts.
But now many of the ads that were being collected in that tool are simply not showing, as Sky News first reported. It means that not only could there be individual ads that would be of interest to researchers but are now hidden, but also that any analysis of the overall ad campaigns will be wrong.
Facebook says that it is urgently looking into the issue. The Ad Library has run into some technical problems before.
Some ads do show up in the library if they are searched for with the specific identification number that is given to each post. But it does not show up when viewed at the level of the catalogue.
Transparency campaigners described the disappearance as a "catastrophic loss of data" that undermines the social network sites' claims to be open through the campaign. Sites like Facebook have argued that they will not fact check political ads but instead allow for transparency in campaigning through tools like the Ad Library.
"Facebook says it is 'urgently investigating' how its Ad Library broke two days before the election," wrote transparency group Who Targets Me on Twitter. "Google’s data is 9 days out of date.
"The big platforms aren’t living up to their word on ad transparency."
"There has been a catastrophic loss of data in Facebook's library, thousands of ads have been hidden," wrote Tristan Hotham, a researcher with Who Targets Me.
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