At first blush, Twitter and Facebook look similar. Each is a social network, connecting users online and presenting them with content in a “feed”, a never-ending list of posts, pictures and videos of pets. Each makes money by selling advertising, and thus has an interest in using every trick to attract users’ attention. And each employs gobbets of data gleaned from users’ behavior to allow advertisers to hit targets precisely, for which they pay handsomely.