In the embryonic, ever evolving era of social media — when milestones come by the day, if not by the second — June 8, 2010, has secured a rightful place in history. That was the day Wael Ghonim, a 29-year-old Google marketing executive, was browsing Facebook in his home in Dubai and found a startling image: a photo­graph of a bloodied and disfigured face, its jaw broken, a young life taken away. That life, he soon learned, had belonged to Khaled Mohamed Said, a 28-year-old from Alexandria who had been beaten to death by the Egyptian police.

At once angered and animated, the Egyptian-­born Ghonim went online and created a Facebook page. “Today they killed Khaled,” he wrote. “If I don’t act for his sake, tomorrow they will kill me.” It took a few moments for Ghonim to settle on a name for the page, one that would fit the…