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 Shares of Millennial Media Inc (NYSE:MM) dropped a harrowing 37% Thursday after the company released mixed first-quarter results and disappointing forward guidance. Millennial also announced its CFO will step down "to pursue other career interests."

So what: Quarterly revenue rose 46.9% year over year, to $72.6 million, which translated to an adjusted net loss of $0.04 per share. Analysts, on average, were looking for a wider adjusted loss of $0.10 per share on higher sales of $75.5 million. Meanwhile, first-quarter adjusted earnings before interest, taxes, depreciation, and amortization came in at a loss of $4.7 million.

That wouldn't have been so bad, but Millennial Media also stated that second-quarter revenue should arrive in the range of $70 million to $75 million, with adjusted EBITDA to be a loss between $5.0 million and $6.0 million.

Finally, Millennial says its executive VP and CFO, Michael Avon, will step down at the end of the second quarter "to pursue other career interests." Avon started his relationship as a venture investor at Columbia Capital, and led Millennial's first round of financing in 2006, then joined as EVP and CFO in late 2009. Now, however, while Avon expressed confidence in Millennial's ability to grow and thrive under its new CEO, Michael Barrett, he states, "I've decided now is the time to move back to my entrepreneurial roots."

Now what: Barrett elaborated on the results, "While I would have liked to have seen stronger performance in the quarter, we have developed an aggressive strategy and I have high confidence that we can execute and capitalize on the growth and promise of mobile."

Today's huge drop may well be an overreaction, but it's also Millennial Media's second plunge in as many quarters following weaker-than-expected guidance. With shares still trading around 56 times next year's estimated earnings -- and keeping in mind those estimates are likely to fall as analysts have time to fully digest today's news -- I personally prefer remaining on the sidelines.

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Steve Symington has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.

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