SAN FRANCISCO — Since the 2016 election, when Russian trolls and a tsunami of misinformation turned social media into a partisan battlefield, Facebook has wrestled with the role it played in President Donald Trump’s victory. Now, according to a memo obtained by The New York Times, a longtime Facebook executive told employees that the company had a moral duty not to tilt the scales against Trump as he seeks reelection. On Dec. 30, Andrew Bosworth, head of Facebook’s virtual and augmented reality division, wrote on his internal Facebook page that, as a liberal, he found himself wanting to use the...