Although we don't believe in timing the market or panicking over daily movements, we do like to keep an eye on market changes -- just in case they're material to our investing thesis.

Stocks finished Thursday on a negative note, with the broadest market measures posting modest declines, while technology stocks helped push the Nasdaq Composite to a slight gain. Amid the causes for concern among investors were poor news reports from several individual companies, with Zynga (ZNGA), Herbalife (HLF -1.69%), and Best Buy (BBY -1.08%) all falling by double-digit percentages today.

Zynga dropped 12% after analysts at Sterne Agee argued that the online video game specialist could disappoint investors when it reports its earnings early next month. The company recently irritated fans of its longest-running video game by deciding to shut down the offering, giving investors another reason to fear that Zynga's management isn't in touch with the wishes of its most dedicated users. With investors already expecting losses throughout 2014 on falling revenue, failing to top a very low bar could lead to a further loss of confidence in Zynga's future.

Herbalife fell 10% in sympathy with Nu Skin Enterprises (NUS -3.40%), which itself sank another 27% today and extended its two-day drop to more than $50 per share. With Chinese regulators at the State Administration for Industry and Commerce beginning an investigation of Nu Skin's business practices, investors fear that China will clamp down on Herbalife's similar distribution model, as well. Although there's no sign that Chinese regulators have actually taken any action on Herbalife, some shareholders aren't waiting to see what results from the Nu Skin investigation before coming to their own conclusions about the continued viability of Herbalife's business in China.

Best Buy plunged 29% after the electronics retailer posted falling same-store sales for the holiday season. Comps fell 0.8%, with a very slight 0.1% gain in international same-store sales failing to offset the 0.9% drop in domestic comps. In one bright spot, comparable-online sales rose 23.5%, and Best Buy managed to improve its market share in some key categories. Yet, falling traffic, especially in the mobile area, along with a big drop in margins, led to the loss of confidence in Best Buy's stock.