The internet of old — composed largely of thousands of scattered communities populated by people who shared interests, identities, causes or hatreds — has been mostly paved over by the social-media giants. In this new landscape, basic intelligible concepts of community become alien: The member becomes the user; the peer becomes the follower; and the ban becomes not exile, but death. It is not surprising that the angriest spirits of the old web occasionally manifest in the new one. But what’s striking is how effectively they can haunt it, and how ill-equipped it is to deal with them.