In his latest guide to startups, Netscape cofounder Marc Andreessen unwittingly offers a rational explanation for Silicon Valley's Facebook frenzy: Sucking up. First, venture capitalists, in their endless neophilia, started using the social networking site. Then entrepreneurs joined in, too, in hopes of impressing those VCs — brazen attempts, in short at brown-nosing their way to getting funded. The same dynamic applies to Twitter, which is an even better medium for elevator-pitching the Valley's short-attention-span financiers. After the jump, Andreessen's analysis.