Left-leaning organizations are dominating political ads on Facebook and Twitter while rightwing advertisers are outspending liberal groups on Google, according to new research.

Computer scientists at the New York University (NYU) Tandon School of Engineering analyzed more than 884,000 recent political ads on the three social media sites. They found that Donald Trump and his Make America Great Again political action committee had the largest number of ads of any candidate while a Republican group called the Senate Leadership Fund spent the most on ads.

Meanwhile, Beto O’Rourke, the Democratic Senate candidate fighting to unseat Ted Cruz in Texas, was the largest spender on Facebook and Twitter, with a focus on ads seeking small donations from outside of his state, the researchers found. In total, ads with political content generated at least 5.9bn “impressions”, meaning the number of times the ads were seen, and cost the sponsors between $116m and $403m.

The report released on Monday is part of NYU’s ongoing ads transparency project and offered a look at social media strategies and priorities of different political groups. The research followed Facebook, Twitter and Google implementing policies and portals this year designed to provide the public insight into political ads.

The Silicon Valley companies have faced mounting pressure surrounding election interference and propaganda polluting the platforms.

The NYU group, which earlier this year found that Trump was the biggest spender of political ads on Facebook, wrote that he remained the “most prolific” political advertiser on the site, in large part because of his tactic of “micro-targeting”, meaning ads aimed at small groups of people. The researchers reviewed nearly 10,000 ads linked to his official Facebook page during a two-week period in September. All of them sought donations, with some promoting rallies or fundraising dinners.

More than 8,000 of Trump’s ads, or 85% of his total, were micro-targeted, meaning they had fewer than 1,000 impressions. This marks a shift away from the traditional model of political ads on television, for example, where large groups of targeted viewers would see the same messaging, the researchers said.

“Depending on who you are, the ads you’re going to see are going to be wildly different,” Damon McCoy, assistant professor and co-author, said in an interview.

“If you see a political ad on Facebook, someone really wanted you to specifically see that ad,” continued Laura Edelson, NYU doctoral student and co-author. With Trump’s high rate of micro-targeting, she added, “That really shows a lot of knowledge about who you are trying to reach.”

The Trump campaign has said that its use of Facebook in the 2016 race helped him beat Hillary Clinton.

Trump’s micro-targeted ads were also geared to older users and men. In terms of age, 24% of impressions were for users 65 and older; 24.5% for those between 55 and 64; and 19.% for users between ages 45 and 54. Men were responsible for 55% of impressions for Trump’s ads.

O’Rourke, who has shattered fundraising records, was one of the few advertisers actively paying for ads on all three sites, the researchers found. He spent $683,000 for a total of at least 23.6m impressions on the platforms, targeting ads in Texas as well as blue states such as Washington, Oregon, Vermont and Massachusetts. He was also the top advertiser on Twitter, based on all metrics.

O’Rourke’s strategy is unique because he doesn’t accept donations from outside political action committees (Pacs), meaning the ads are all directly funded by his campaign.

The majority of spending on Google, however, did come from Pacs, which typically have large budgets and were responsible for 61% of ads on the search engine, costing $3.1m. On Facebook, Pacs accounted for 23% of spending, which cost $1.2m. Pac ads tended to be more negative and focused on opponents, noted Edelson.

On Facebook, the NextGen Climate Action committee, an environmental advocacy group, was a top advertiser in terms of number of ads, spending and impressions. Priorities USA, a Democratic Super Pac, and the National Republican Congressional Committee, also spent big and had large reaches on the platform.

The top three Google spenders were GOP groups – the Senate Leadership Fund, Congressional Leadership Fund and Americans for Prosperity.

Senator Kamala Harris, floated as a potential Democratic presidential candidate, was in the top 10 for total number of Facebook ads. The Democratic senator Kirsten Gillibrand, also mentioned as a possible White House candidate, was one of the top spenders on Twitter.