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

What: Shares of Internet advertising company ValueClick (Nasdaq: VCLK) plummeted 25% on Thursday after its quarterly results and outlook disappointed Wall Street.

So what: ValueClick's first-quarter profit breezed past estimates, but a big top-line miss -- $152.9 million versus the consensus of $157.8 million -- coupled with a weak second-quarter outlook is triggering serious concerns over slowing growth. The shares have rallied nicely over the past several months, but the results are forcing Mr. Market to quickly sober up.

Now what: Management now sees second-quarter revenue of just $155 million to $160 million, well below Wall Street's view of $168 million. "Given the success of our efforts to reduce the paid traffic component of our Owned & Operated segment in the past two quarters while improving this segment's margin profile, we will accelerate this process in the second quarter," noted CFO John Pitstick. "We expect this will result in a sequential revenue decrease in the O&O segment in the second quarter ... but with continued gross margin improvement." So while ValueClick's slowing growth is certainly a turnoff for momentum investors, its improving profitability might be a good sign for longer-term owners.

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