Alex Gilbert, 27, who works for a nonprofit in Houston that teaches creative writing to kids, is still puzzling over an old friend—"a particularly masculine-type dude"—who plays in a heavy-metal band and heads a motorcycle club yet posts videos on Facebook of "uber cute" kittens. "It's not fodder for your real-life conversation," Mr. Gilbert says. "We're not going to get together and talk about how cute kittens are."Consider,how people you know often seem different online—displaying sides of their personalities you have never seen before.