Washington • It cannot be a sign of social health that the number of tweets per day worldwide exploded from 5,000 in 2007 to 500 million six years later. And this might be related, by a few degrees of separation, to the fact that whereas in the 1992 presidential election more than one-third of America’s 3,113 counties or their equivalents had a single-digit margin of victory, in the 2016 presidential, fewer than 10 percent did. And to the fact that in 2016, 1,196 counties — about 2.5 times the average over the preceding 20 years — were decided by margins larger than 50 percent.