It had existed as a phrase for some time, but it wasn’t until two black men died in the summer of 2014 that Black Lives Matter began to flicker to life as a Twitter hashtag. The roots of the phrase are commonly traced to a July 2013 Facebook post by Alicia Garza, a California-based activist, but it appeared in the Twitter-friendly form #BlackLivesMatter only in fits and starts over the course of the following year, according to a Pew Research Center analysis on race and social media released this month. The hashtag had a small, but sustained increase in use...