Anyone in the art market who was not already been paying attention to the social media platform Instagram had to sit up and take notice in April after the actor Pierce Brosnan visited the showroom of Phillips auction house in London. Mr. Brosnan snapped a selfie in front of a work he admired: the “Lockheed Lounge,” a space-age aluminum chaise longue by the industrial designer Marc Newson. Then he added the words “let the bidding commence,” and posted it to the 164,000 followers of his Instagram feed.