REUTERS - Snapchat shares plunged ~40% and sparked a sector-wide selloff after a profit warning from the parent signaled tough times ahead for the once-booming digital ad industry. The company was on track to lose $15B in market capitalization, while shares of major online advertisers and social-media firms were set to lose a combine $200B in value from the rout. Snap shares closed at $12.79, lower than their 2017 IPO price of $17.