THE INTERNET has matured, and yet even so is still evolving, with the latest killer application being social engagement—a development paralleled by the rising interest in community and connection throughout society at large. These days, the best way to sell something is not to engage consumers with a brand, but to host or facilitate the ability of people to engage with one another. While they’re doing so, they’ll do business as well. The dynamic of social engagement belies some of the earliest concerns about the Internet. Instead of isolating people into private experiences shared with no one else or clustering people into closed communities of narrow interests, the Internet is bringing people together in new, unprecedented ways. The...