Bubblicious. Mark Andreesen is making big bets on the next tech IPO rally.

Over a six-month period, Mr. Andreessen tapped his formidable network of Silicon Valley connections to snag stakes in Facebook Inc., micro-blogging service Twitter Inc. and deals-site Groupon Inc. Other investments include social-game developer Zynga Inc. and Internet-telephone company Skype SA. In the process, he helped to ignite Silicon Valley's latest Web boom and the burgeoning market for private-company shares.