With just two words — “substantially stretched” —
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Federal Reserve Chairwoman Janet Yellen sent social media stocks tumbling on Tuesday and stirred rampant talk of a “bubble” in the market.

The assessment, a part of the central bank’s annual report to Congress, was couched in technical language, warning that “risk-taking” in the social media and biotechnology sectors appeared to have increased, unlike in other industries where stock valuations have kept in line with historical averages.