In the old days, companies went public “early and often,” Marc Andreessen told a group of reporters at IAB’s Mixx conference today. A company could IPO when it had hit just a few million dollars in revenue, leaving much of the value to be captured by public market investors.

But the stock market has changed since his days, making it more difficult for companies to go public. Andreessen advises portfolio companies to do so only when they’re in a position of strength — when they have the product figured out, the patent protection in line, the client relationships shored up.

The trend is bad for investors — they miss out on the value created while companies are private. But it’s good for companies. “It’s easier to function as a good, young company when you’re private,” he said. Then they go public when they’ve built “a fortress,” he said.

One of his most well known portfolio companies, Facebook, thought it had a fortress. That’s changed since going public, and Mark Zuckerberg and team have learned they need to sell themselves a little more. That’s why Andreessen was at IAB’s Mixx conference alongside Facebook COO Sheryl Sandberg and VP of Marketing Carolyn Everson as part of Advertising Week. Project Boost Facebook’s Stock was in full effect. It’s working…sorta.

During a live interview, Charlie Rose made an off-hand comment about Facebook’s $30-some-billion market cap. The crowd laughed — Sandberg did not. Also, Andreessen noted that we’re in an era where tech stocks have not traded this low relative to industrial companies in years.

This led to the classic “are we in a bubble?” conversation. Andreessen has not been shy about expressing this point of view. “We’re not in a bubble, not even a trace of a bubble. It’s the opporsite of the bubble,” he said. “Everybody worries about the bubble when there’s no bubble. No one was saying there was a bubble in ’99. Everyone was panicking that they’d get left behind.”