Whenever I even suggested the word “bubble” in my reporting, I became a punching bag. After I scrutinized the ethics (and preposterous valuation) of Path, an ill-fated social network, Michael Arrington, once a nexus of power in Silicon Valley who had invested in the start-up, called me a “pit bull” and said I wasn’t a very noble person. But lately the worries have spread. There are now fast approaching 100 unicorns based in the U.S. alone, and counting. The NASDAQ recently closed at an all-time high, surpassing a record set right before the dot-com crash in 2000. The Shiller P/E ratio, a measure of the ratio of price to earnings, has a number of investors worrying, with The Wall Street Journal noting that it shows stocks are “frothy.”