Local politicians and authorities are targeting constituents' Facebook feeds to influence opinions of neighbourhood issues.
But a new investigation has found hundreds of adverts placed without declaring who paid for them, in breach of the site's transparency rules.
Canvassing is increasingly moving from the streets to the web, with more than 10,000 ads placed by MPs, councillors, elected officials or local authorities in less than a year.
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.
Facebook said their "industry-leading tools" make spending on political ads more transparent.
But critics say told us it is still too easy to run such campaigns without declaring who is funding them.
Hundreds of ads breaching Facebook transparency rules
It has been almost a year since Facebook changed its rules so anyone placing a political advert was required to declare who paid for it.
But an investigation by the JPIMedia Data Unit identified around 300 ads on the pages of local politicians and councils which were run without these disclaimers.
This includes 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 identified and removed by Facebook, but some were seen hundreds of thousands of times before being deleted.
Adverts placed by politicians, parties, lobby groups and others can be viewed by anyone through the Facebook Ad Library.
While this shows basic analytics about the types of people who saw each advert, it does not reveal how that demographic was targeted.
Data and Democracy Officer at Open Rights Group, Pascal Crowe, warned that this "key battleground for political campaigns" needs better regulating.
“It also demonstrates that the rules that shape our elections are ripe for reform," he said.
"For example, it is currently too easy to field a political advert on Facebook without revealing who is paying for that ad.
“It is now perhaps easier than ever to game the system and avoid being held to account."
Political advertising in the digital age
The Cabinet Office told us they are also working on making digital political advertising more transparent.
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.”
Facebook said people could report any concerns to themselves or regulators. Image: Shutterstock
A spokesperson for Facebook added: "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."
Case study: The Mayor of London's Facebook page
Promotional images of Mayor of London Sadiq Khan donning hard hats, planting trees or meeting the public have been placed in Facebook users’ feeds millions of times.
Since October last year, the Greater London Authority has spent more than £53,000 on Facebook adverts placed through the Mayor of London page - the UK’s highest spending mayoral account by some distance.
Some ads give public information messages while others promote the authority’s work.
One advert about how the authority was helping the homeless ran without declaring who paid for it, in breach of Facebook’s rules. The social media giant removed the ad, but not before it was shown on people’s feeds at least half a million times.
There is no suggestion that those behind the advert had meant to deceive users.
A spokesperson for the Mayor said: “London has a significantly bigger population than the UK cities represented by other Mayors, and therefore spends are not comparable.
An example advert placed by the Greater London Authority on the Mayor of London's Facebook page. Copyright: Greater London Authority
“The Mayor has a responsibility to keep Londoners informed and engaged around our work, including informing them of services that are available and policies which may impact them.
“Facebook is one of a range of digital channels which we have used to communicate around key issues such as tackling youth violence, supporting rough sleepers and improving air quality.”
The spokesperson said adverts placed through the Facebook page were non-political.
Mr Khan has a separate politician’s page, which, according to the Facebook ad library, has carried at least £400 worth of ads paid for by the London Labour Party.
More than £24,000 spent promoting councillors
With 40 million users in the UK alone, Facebook is seen by many politicians and groups as an effective way of reaching their constituents, and potential voters.
At the time of analysis at least £6.4 million had already been spent on political advertising through the social networking site.
Since last October, at least £209,000 has been spent on ads for individual MPs - including campaigns for the Conservative Party’s leadership contest.
At least £32,000 was spent on ads placed through MEPs’ Facebook pages and at least £9,000 has been spent on the pages of MSPs.
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.
The Facebook Ad Library is being constantly updated with daily advertising spends.