The Conservatives halved the amount of money they spent on Facebook ads at last year's general election, new figures show.
The party spent £2.1m on the social media platform at the 2017 election compared with £1m in 2019.
Overall, Boris Johnson's party spent £16.5m on campaigning - £2.1m less than Theresa May in 2017.
The Electoral Commission has not yet released spending figures for Labour and the Lib Dems.
The watchdog said its normal reporting process had been delayed by the coronavirus pandemic and it would be releasing spending figures for the other parties as soon as they are ready. It did not give a date for the next batch of figures.
The figures released on Wednesday showed that the Scottish National Party spent just over £1m and Plaid Cymru spent £183,914.
The Independent Group for Change, whose two candidates, former Tories Heidi Allen and Anna Soubry, lost their seats, spent £29,556.
The UK Independence Party saw a dramatic drop in spending, from £273,104 in 2017 to £8,761 last year.
Figures from the Brexit Party and the Green Party have also yet to be published.
Several non-party groups campaigning for and against Brexit are included in the latest release.
On the Remain side, Gina Miller's Remain United tactical voting campaign spent £199,581. Best for Britain, which also ran a tactical voting site, spent £422,498 and Led By Donkeys, a crowdfunded campaign, spent £458,237.
On the Leave side, City financier and former Tory donor Jeremy Hosking spent £484,248.
Momentum, the campaign group set up to support Jeremy Corbyn's leadership of the Labour Party, spent £500,458.
Further figures from non-party groups are expected in the coming weeks, when a fuller picture will emerge.
Like all the big parties, the Tories use Facebook to micro-target voters in marginal seats with hundreds of specially-tailored messages - in 2019 these were often about Boris Johnson's plans for the NHS.
But there appears to have been a switch back towards broader, less targeted online advertising last year.
The Tories spent £1.2m on Facebook ads in 2015, helping the party to secure an unexpected Commons majority. They ramped up spending on the site in 2017, a year in which they lost their Commons majority.
The party continued to increase the share of its advertising budget spent online last year, which accounted for 63% of £3m spent.
But it reduced the amount spent on Facebook ads and increased its spending with Google from £562,154 to £879,091.
The Tories are reported to have spent more than any UK political party in history with YouTube, which is owned by Google, in the run up to polling day, including banner ads on the site's home page.
But the party spent more on unsolicited mail to voters - £5.9m - than it did on advertising, the figures show.
The Electoral Commission figures also reveal how Tory workers fed themselves during the campaign, with £22,735 spent with food delivery service Uber Eats, £1,972 on Pizza Hut, £1,867 at Tesco and £1,492 at Marks and Spencer.