Is social media a waste of time, as David Remnick said? Maybe so. But Twitter, Instagram and the rest make keeping up with other people's lives easy and free. Where the Cannes festivalgoer is concerned, a spur-of-the-moment video on Instagram by a filmmaker can potentially add insight into what they're experiencing. At the same time, it's important not to lose touch of the fact that social media is often as vapid as it is useless