In 2005, in a bustling University of Virginia library lobby better suited for socializing than studying, Reddit entered the world humbly. A small plaque now hangs next to the clunky desktop computer where Reddit’s cofounder Alexis Ohanian, then an undergrad at the state school, registered the now-ubiquitous domain name. It marked just another day for dozens of students printing essays nearby, but whether they realized it or not, they were witnessing history: The birth of a social media site that would send Wall Street into crisis mode (and make billionaires out of a few lucky investors) 16 yea...