Joe Biden’s spend on Facebook advertising dropped an astonishing 65 percent last month after a costly push for elusive young voters saw the Democratic nominee pumping up to $1.4 million a day into the platform. With less than 100 days until the election, the campaign slashed its entire digital spend by more than half between June and July, according to ad records from Facebook and Google. Digital operatives speculated the campaign’s decision to cut its online drive, which came at the height of the Black Lives Matter protests in early June, was because the ads were unsuccessful and failed to...