THE amount spent by political parties in Scotland on Facebook advertising soared by half in just one year, the Sunday Herald can reveal, as concerns grow over voters being targeted by "dark adverts" on the social media site.

An analysis of electoral spending shows nearly £48,000 was spent on advertising on the social media site north of the border during the UK General Election campaign in 2015.

That figure increased to spending of £70,635 during the Scottish Parliament elections last year.

Unlike the costs associated with leafleting campaigns, or time needed to go visit people on the doorstep, thousands of voters can be targeted with individual advertising content such as videos or messages for relatively low cost.

For example, the Sunday Herald has learned the cost of reaching 18,000 constituents with a targeted post is as little as £70.

It comes as the influence of the social media giant on elections is coming under increased scrutiny. Facebook uses closely guarded computer algorithms to track users’ lives, allowing it to sell precisely-targeted advertising.

However, concerns have been raised over the extent to which political advertising on Facebook is now being used to influence elections, following reports that pro-Leave groups paid millions of pounds to a company to collect data about users and target them with personalised messages which only they can see – sometimes called "dark advertising".

A BBC Panorama investigation last week also revealed how the Leave EU campaign claimed Facebook had been a “game-changer” in persuading voters to back Brexit. Political strategist Gerry Gunster told the programme how it allowed them to “micro-target” different types of people in different parts of the UK.

Jim Killock, executive director of Open Rights Group, an organisation which campaigns for online privacy, said: “The power of profiling and corporate surveillance is becoming better understood as this power is being utilised not just to sell fizzy drinks and T-shirts but to influence elections.

“We all need to be very aware of how our information is being used, and of attempts to manipulate us.

“There is tremendous power in the hands of a tiny number of internet companies who have profiled billions of online users.”

The analysis by the Sunday Herald shows the top spender on the social media site during the 2015 General Election – in which the SNP recorded a landslide victory north of the Border – was the Conservatives, followed by Labour.

The Conservatives election spending vastly exceeded any other party, at £27,804.

The next biggest spender was Labour at £6,946, followed by Ukip at £5,808, the SNP at £5,466 and the Liberal Democrats at £1,948.

In the Scottish Parliament elections the following year, while Labour and the Conservatives spent roughly the same on advertising, it was the Conservatives who beat Labour to become Scotland’s second party behind the SNP. Labour spent the most on Facebook at £22,672 followed by the Tories at £22,527. The Greens spent £12,907, the SNP £6,093 and the Liberal Democrats £5,205. Ukip spent the least at £1,228.

The total for all parties accounted for seven per cent of the amount spent on advertising.

The Sunday Herald asked all the parties a series of questions including their response to concerns over "dark advertising" and how much they intended to spend on Facebook advertising in the forthcoming General Election. The SNP, Greens and the Liberal Democrats did not respond.

A Scottish Conservative spokesman said: “Social media is an important tool for communicating with voters, and the party will be using it to help spread our message that we don’t want the SNP’s second referendum on independence.

“However the main way we engage with voters is through our candidates speaking to Scots up and down the country face to face about their concerns, and about what they want from their local MP.”

A Scottish Labour spokesman said: “Social media, including Facebook, is an increasingly valuable campaign tool.

We will use online platforms and advertising to take our message of a government for the many and not the few to voters across the country.

“Our policies – including investing in the NHS and education, and standing firmly against a second independence referendum – will feature prominently on platforms such as Facebook."

David Coburn, leader of UKIP Scotland, said: "Social media is a great platform to convey your message. Some people may only use the internet for political information, others may not. I have not used Facebook to target the electorate specifically – Facebook has been a platform where my followers merely keep up to date with my work.

“However, it can be an essential tool for campaigning and my team are currently researching the pros and cons of social media campaigning."

Facebook did not respond to a request to provide comment on the concerns raised over "dark advertising" and regulation, and information on what services it can provide to political parties.