Shares of Naver Corp., South Korea's top Internet portal operator, dropped more than 8 percent on Thursday on concerns that its free chat service LINE would face increased competition following Facebook Inc.'s takeover of WhatsApp, the largest free messenger service provider in the North American region with a user base of 450 million. Naver sank 8.13 percent to close at 689,000 won (US$642) on the Seoul bourse after falling to as low as 687,000 won at one point. Its market cap stood at 22.7 trillion won.