In early afternoon trading in New York, Facebook was 5.5 per cent lower, at $98.33, while Salesforce was 8.5 per cent lower, at $53.53.
That dragged the Nasdaq down 2.9 per cent, to 4,237 points.
Twitter's shares fell under $15 for the first time ever as the market opened after news emerged it is considering changing its algorithm. However, they later rallied slightly, to $15.15 - although it was still 3.5 per cent below yesterday's closing price.
On Saturday reports from Buzzfeed suggested Twitter was planning to roll out an "algorithmic" timeline as early as next week, which will show tweets based on what users like, rather than in the current chronological format.
Other tech stocks fell, including Etsy, which dropped 5.3 per cent to the decidedly sinister price of $6.66, and Groupon, which fell 5.9 per cent to $2.30.