Joseph R. Biden Jr.’s presidential campaign has sharply scaled back his online advertising, cutting spending so severely since August that he is now investing only a fraction of what his top rivals are on Facebook and Google, the two dominant internet platforms.

In a race where many voters are following politics on their smartphones, Mr. Biden’s pullback is an unusual and potentially worrisome sign about his appeal among the Democratic activists, young people and donors who are especially engaged on social media. Candidates rarely withdraw so much money from their online campaigns unless they are seeing weak results in online fund-raising, according to interviews with digital strategists.

Mr. Biden and other candidates will start announcing fund-raising results as early as Tuesday morning for the third quarter of 2019; he came in second for the last quarter, behind Pete Buttigieg, pulling in online money early but mostly relying on big donors. More recently, Senator Elizabeth Warren of Massachusetts and other top rivals have outspent Mr. Biden online: He was only the sixth-biggest Democratic spender on Facebook ads in the seven days ending Saturday, even as he began spending more money on digital ads targeting President Trump, Ukraine and impeachment.