How will Facebook celebrate its 20th birthday? Perhaps it will create one of those cute video montages they like to generate at significant moments. Starting with a tinkling piano soundtrack, a couple of breathless friend requests, and some self-conscious, tentative writing of “hello!” on other users’ walls, it might then pass quickly through moments of chronic oversharing, passive-aggressive, stalking of exes, and horrified untagging of yourself in unflattering photos.