Most of those companies don't derive the bulk of their revenue from consumer advertising.
ADDENDUM: The next post in this thread includes a link to a FINANCIAL TIMES article that states: "Fears that Google could become a victim of a US economy flirting with recession appeared to be the main factor in the share sell-off."
Really? I'd say the opposite is true. They willingly gave up short-term ad revenue when they introduced AdWords quality scores, for example, and again when they reduced the AdSense click area to discourage accidental clicks. Everything I've seen suggests that they're more into long-term strategy than short-term tactics. (This thread [webmasterworld.com] is a good case in point.)
Of course, people who think like day traders may have trouble seeing the bigger picture.