Hundreds of websites — including those of biggies such as Netflix, Twitter and Spotify — on Friday fell prey to massive DDoS attacks that cut off access to Internet users on the East Coast and elsewhere across the United States. Three attacks were launched over a period of hours against Internet performance management company Dyn, which provides support to eight of the top 10 Internet service and retail companies and six of the top 10 entertainment companies listed in the Fortune 500.