US President Donald Trump's re-election campaign has spent far more than any single Democratic presidential candidate on Facebook advertising, reprising a strategy that was central to his 2016 victory.

Trump spent particularly heavily on Facebook ads at the beginning of the year, when the number of Democratic candidates was smaller. But the playing field has shifted.

Since entering the race late last month, former Vice-President Joe Biden has pumped more than US$1 million into Facebook ads, outspending Trump's campaign for three of the past four weeks.

So far this year, Trump has spent about US$5 million on Facebook advertising. Early