Google's Eric Schmidt sat down for an hour-long chat with All Thing D editors Kara Swisher and Walt Mossberg last night at the kick off to the D9 conference outside of Los Angeles.

Besides facing questions about privacy and U.S. antitrust investigations, the executive chairman and former CEO had some interesting comments to make about rivals, his company's failure to go social, and what he believes is a "gang of four" driving innovation and growth in the industry.

On the latter, Schmidt noted that there are four companies exploiting platform strategies very well, allowing other companies to build additional value on top of their services to the benefit of both consumers and shareholders. This gang of four, according to Schmidt, includes Google, Apple, Amazon, and Facebook.

The combined value of these four companies is more than half a trillion dollars, something unprecedented in the history of tech were single, large platform companies used to dominate -- IBM through the early 1980s, followed by Microsoft. He omitted the latter from his list because it doesn’t drive the consumer market anymore and is largely an enterprise play. Even though they have succeeded with the Xbox, Microsoft main business is still by far Windows and Office.

Furthermore, Schmidt believes that a shakeup among the gang of four could come at any time, as there's an inevitable slow-down when companies get big, middle aged and boring. Product innovation is absolutely required to avoid this and in his view Twitter and PayPal could be possible gatecrashers into the select group.

Schmidt also took responsibility for the search giant's failure to counter Facebook's explosive growth in social networking, saying he saw the threat coming but failed to counter it. He gave credit to Facebook for mastering "identity" on the web and said they tried very hard to work on a search partnership, but Microsoft ultimately won that deal.