The Google-Facebook duopoly made up 67.8% of the UK digital ad market last year, but we expect that figure to drop slightly to 65.9% this year.

As the pandemic disrupts Google's ad business, Facebook will fare better and continue to growth. However, despite the slight dip this year, the duopoly's share will rebound to 69.1% in 2021, nearly in line with our pre-pandemic forecast.

Google's overall ad revenues will fall 5.5% this year due to a sharp fall in search revenues, though display will continue growing. Major pullbacks in travel ad spending and a shift to noncommercial search activity during the pandemic led to the decline from last year's 14.5% growth, according to eMarketer forecasting analyst at Insider Intelligence Peter Vahle.

Some parts of the company's ad business will be hit harder than others: For example, Google's share of the UK search advertising market has already been falling, and we expect the pandemic will accelerate this trend, for a total decline of 7.4%. But the company's display ad revenues will still grow 3.8% this year, as display is better suited for the direct-response advertising that many marketers are turning to in the pandemic, and YouTube's relative strength helps offset losses elsewhere.

Facebook's ad revenues will grow by 5.6% this year, a major drop from our pre-pandemic estimate of 19.2%. The company's direct-response advertising options, in addition to its overall familiarity to advertisers and ease of use, help to make it more resilient to ad market volatility. Still, this growth will not be enough for Facebook to gain share in the display ad market against faster-growing digital companies.

Last year, Facebook made up a 60.0% share of the market, but we expect a slight decrease to 59.1% this year. Instagram's strong growth has helped the company maintain this share, but declines to the core Facebook platform will weigh it down.