The Battle of Poitiers, which took place on September 19, 1356, was one of the most important acts of arms of the  100 Years' War, which pitted the kingdoms of France and England against each other.

I remember the Sunday I hit 37 matches on Facebook Dating—then watched them flat-line because my opening line was “hey.” Cringe, right? Here’s the thing: how Facebook Dating works isn’t the mystery; getting people to hit “Like” back is. If your inbox feels like a ghost town, you’re not alone. After...