Shares of the largest publicly listed US crypto exchange Coinbase slipped in premarket trading on Monday. After a surge in traffic following a Super Bowl advertisement resulted in the app crashing briefly over the weekend. A floating, colorful QR code was displayed during NFL’s title game, the biggest US TV event of the year. It redirected people to a link that offered $15 in bitcoin to those who sign up for a Coinbase account before Feb. 15. The site saw 20 million hits on its landing page in one minute after the launch of the bizarre advert. Coinbase reported “more traffic