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 "daily deal" specialist Groupon (NASDAQ:GRPN) plummeted 19% today after its quarterly results and outlook missed Wall Street expectations.

So what: Groupon's fourth-quarter revenue managed to grow 30%, but a surprise loss of $0.01 per share -- versus Wall Street's view of a $0.03 profit -- coupled with downbeat guidance reinforces the belief the daily deal market is rapidly deteriorating. While market share and gross billings grew during the quarter, management had to lower its "take rate" and sacrifice profitability to do it, underlining concerns over Groupon's competitive position, as well.

Now what: Management now sees first-quarter revenue in the range of $560 million and $610 million, a year-over-year increase of 0% and 9%, which is well below the average analyst estimate of $650 million. "We will continue to invest in growth through 2013 as we see new opportunities to give our customers what they want," CEO Andrew Mason reassured investors. But until those opportunities translate into visible margin expansion, expect the stock to remain under heavy pressure.

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