Yes and no. If the companies in question are privately held then to a certain extent the risk is only with investors brave (or foolish) enough to back them. However, what do you think would happen to the tech industry, and potentially the wider world economy, if a big name company suddenly went up in smoke, public or otherwise?

Now, please pause for a moment and consider: it doesn't have to be Microsoft or Apple or Oracle. Some of the crazy valuations are putting much less robust companies within an order of magnitude. If one of them goes, it's going to be big news and stock prices right across hi-tech industry could race to the bottom scarily fast as we've seen before.

If you are, for example, Zynga, then the kind of disaster that could lead to catastrophic failure seems awfully difficult for you to anticipate or control. Likewise all kinds of new companies being built on Twitter and so on, keeping in mind that Twitter (as far as I know) doesn't actually make any money itself yet and could easily be displaced by the next new shiny thing tomorrow.

I'm not saying it's going to happen at all, never mind tomorrow, but there is definitely a house of cards building up here, and of such things, market bubbles are made.

The two leverages have another thing in common - risk.