Hundreds of individual MPs, elected officials and local authorities have placed nearly half a million pounds’ worth of promotions on the site in less than a year, we can reveal.
In the wake of the Cambridge Analytica scandal, the social media giant last year began publishing details of who places - and pays for - adverts promoting political or social issues.
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Spending on these ads, often targeted to specific groups, has totalled more than £6.4m since last October.
And while much of the attention has focused on the big parties and pressure groups on either side of the Brexit divide, the data also reveals the thousands of adverts placed at a local level, often seeking to influence constituents on what can appear to be seemingly mundane neighbourhood issues.
Under new rules Facebook introduced in October 2018, anyone placing a political advert must declare who paid for it.
Our investigation identified around 300 ads on the pages of local politicians and councils which were run without these disclaimers - including 40 placed on behalf of sitting MPs.
There is no suggestion that any of the adverts had been deliberate attempts to deceive constituents. They were all found and removed by Facebook.
But with a general election looking likely in the coming months, campaigners have questioned the transparency of the system.
The Open Rights Group, which campaigns for internet users’ digital rights, said social media has become a “key battleground for political campaigns”.
Its data and democracy officer, Pascal Crowe, said the “rules that shape our elections are ripe for reform”.
“For example, it is currently too easy to field a political advert on Facebook without revealing who is paying for that ad,” he said.
“It is now perhaps easier than ever to game the system and avoid being held to account."
A spokesperson for Facebook said: "Our industry-leading tools are making it easier to see all political ads on our platforms, and archives them for seven years in Facebook's Ad Library.
“People are able to report concerns to us or regulators as appropriate."
A Government spokesperson said: "There should be greater transparency in political advertising, which is why we have already pledged to introduce the requirement for digital election material to be clearly branded. We will bring forward technical proposals by the end of the year.”
With 40 million users in the UK alone, Facebook is seen by many politicians and groups as an effective way of reaching constituents.
Since last October, at least £209,000 has been spent on ads for individual MPs - including campaigns for the Conservative Party’s leadership contest - and at least £32,000 was spent on ads placed through MEPs’ Facebook pages.
Local councils spent more than £20,000 on Facebook promotions, while adverts placed by individual councillors totalled more than £24,000.
Around £120,000 was spent on promotions for elected mayors and mayoral candidates.