Millennial Media shares tumbled 40 percent at the opening bell Thursday on news that its losses more than tripled in the first three months of 2014, despite a 47 percent surge in revenue year-over-year.

The Baltimore-based mobile advertising firm also announced its chief financial officer, Michael Avon, will step down at the end of June "to pursue other career interests." His departure comes six months after Paul Palmieri, the company's founder and CEO, left the company to join a venture capital firm.

Millennial lost $12.9 million on $72.6 million in sales in the three months ended March 31, compared with a $3.8 million loss on $49.4 million in sales in the same quarter of last year.

The loss per share amounted to 12 cents, compared to 5 cents in 2013's first quarter. The results fell short of Wall Street analysts' estimates of a loss of 10 cents per share for the first quarter.

Shares opened at $3.19, down from a close of $5.35 Wednesday afternoon. Shares had fallen by nearly half, to below $3, in after-hours trading Wednesday night.

Michael Barrett, who succeed Palmieri as CEO, said the losses were driven by increased competition and declines in its app download business.

"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," Barrett said in a statement.

Avon had served as executive vice president and chief financial officer since fall 2009, helping lead the company through its 2012 initial public stock offering. Before joining the company, he led its first round of venture capital financing as a venture investor at Columbia Capital.

Millennial has begun a search for his replacement; Avon will serve an advisory role during the transition.

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