Regulators must find a way of monitoring and addressing the way political advertising on Facebook creates new types of inequalities for campaigners, experts have said.
The online platform is increasingly used during elections, but it is impossible to track the impact of political advertising and how politicians are using this tool.
The Electoral Commission has called for a change in the law so political adverts are marked clearly with the name of whoever has paid for them. But a new study suggests regulation must also take into account how Facebook algorithms mean the same advertising spend has different results.
Researchers say any new regulations must show more understanding of the differences in online campaigning, and do more to capture the true spend on political adverts on Facebook.
Traditional campaigning regulations are based on the theory that spending by each political party leads to a similar result - for example if political parties spent the same amount on leaflets the literature would reach a similar number of people.
The research, published in Political Quarterly, by Dr Katharine Dommett, from the University of Sheffield and Dr Sam Power, from the University of Exeter says this cannot apply to Facebook advertising, where the impact is dependent on the audience the advertiser wants to reach. This means different spend will have different results. Adverts in a marginal constituency will be more expensive, as will adverts that are directed at an audience that is in high demand from advertisers.
Dr Dommett said: "As digital political campaigning grows it is now increasingly difficult for existing regulators to capture the true extent of what is happening online, let alone whether these practices violate democratic norms. The unreliability of existing data on the use of Facebook needs to be acknowledged by regulators if campaigning spending is to be effectively interpreted and understood.
"The lack of clear information should concern anyone responsible for overseeing the conduct of modern campaigns."
Dr Power said: "Although Facebook has introduced some new transparency measures, nobody can fully monitor both how it is being used by political parties and the inequalities of access they can face. It is also not Facebook's role to regulate elections. We need to recognise these limitations to think about whether and how existing reporting requirements need to change.
"Regulators around the world need to think about how to monitor and respond to spending principles that are creating inequalities in the electoral market place."
As part of their study Dr Dommett and Dr Power looked in detail at spending on Facebook by political parties. There has been a significant rise in spending on digital advertising in recent years. In 2014, only £30,000 - 1.7 per cent of the overall advertising budget - was spent on online advertising, yet by 2017 this figure had risen to £4.3 million - 42.8 per cent.
Party spending returns also show that just over £3.16 million was spent on Facebook advertising by all UK parties at the 2017 general election, compared with just over £1 million on Google, £54,000 on Twitter, just under £25,000 on Amazon and just £239,000 on 'traditional' advertising in national and regional media outlets.