Bill Gates Mike Nudelman/Business Insider Half of Silicon Valley start-ups are “silly” and the price paid by Facebook for WhatsApp was “higher than I would have expected”, says Microsoft co-founder Bill Gates.

“Innovation in California is at its absolute peak right now. Sure, half of the companies are silly, and you know two-thirds of them are going to go bankrupt, but the dozen or so ideas that emerge out of that are going to be really important,” he said in an interview with Rolling Stone.

Gates, who has a fortune of more than $76 billion, recently returned to Microsoft as a “technology adviser”, where he expects to spend a third of his time, with the rest devoted to his charity work with the Bill and Melinda Gates Foundation which he runs with his wife.

"Satya [Nadella, Microsoft chief executive] has asked me to review the product plans and come in and help make some quick decisions and pick some new directions," he said.

One area of focus will be the company’s desktop applications: “Office and the other Microsoft assets that we built in the Nineties and kept tuning up have lasted a long time. Now, they need more than a tune-up. But that's pretty exciting for the people inside who say, ‘We need to take a little risk and do some new stuff’.”

Commenting on Facebook’s recent $19 billion acquisition of WhatsApp, Gates admitted that Microsoft “would have been willing to buy it” but not at the price paid by Mark Zuckerberg.

“I think his aggressiveness is wise – although the price is higher than I would have expected. It shows that user bases are extremely valuable,” he said.

“We're both Harvard dropouts, we both had strong, stubborn views of what software could do. I give him more credit for shaping the user interface of his product. He's more of a product manager than I was. I'm more of a coder, down in the bowels and the architecture, than he is. But, you know, that's not that major of a difference. I start with architecture, and Mark starts with products, and Steve Jobs started with aesthetics.”

He said that one of the biggest surprises about how technology has advanced is that there are “less robots than I would have guessed” and that “vision and speech have come a little later” but that these industries will probably take firm root within five years.