Investors Flocking to Web Networking Sites By MICHAEL LIEDTKE    SAN FRANCISCO (AP) -- Internet whiz kids Marc Andreessen, Josh Kopelman and Joe Kraus share something in common besides reaping huge jackpots during the dot-com boom.    All three belong to LinkedIn, a rapidly growing online networking service whose ability to connect people with common friends and work interests helped persuade the trio to invest in the company before it even started to generate revenue...