U.S. stocks fell for a second session on Monday, with the Nasdaq especially hard hit as investors shed Facebook, Google and other high flyers.

"The Nasdaq seems to be getting hit disproportionately hard. I presume some of that is due to weakness in biotech," said Matthew Kaufler, portfolio manager at Federated Investors.

The Nasdaq dropped 50.40 points, or 1.2 percent, to 4,226.39, with the index knocked by heavy losses among some if its recent outperformers, including Facebook, Tesla Motors and Netflix.

"These are all stocks that have had enormous gains over the last year; those are high-valuation stocks, and when you talk about a risk-off trade where people are withdrawing capital from riskier areas, that where people pull from first, stocks that have had significant run ups," Kaufler said.

"Tesla is a stock we would never look at because of its multiple. We're a value shop, we would pay for growth, but not that much," Kim Forrest, senior equity analyst at Fort Pitt Capital, said of the electric-car maker's valuation.

An article in the Wall Street Journal about advertisers not knowing if click throughs generate leads "could be pushing Google down," Forrest added. Shares of the search engine were off 1.7 percent.

The internet "can be a place that is ripe for fraud," so if a substantial portion of people on Facebook are fake, for instance, advertisers "would not be happy," Forrest added of the social-networking site, down 2.7 percent.