An activist with a 10 percent stake in Pandora is pressuring the company to sell itself

Shares of Pandora are up almost four percent in after-hours trading, after an activist investor launched a campaign to force the company to put itself up for sale.

In a letter addressed to Pandora's board of directors filed on Monday, Corvex Management's Keith Meister claims that his firm owns 9.9% of the company's stock, and expresses concerns "the company has been unable to date to translate its great product into a great business with an attractive public market valuation."

With Pandora's share price sagging, with big spending on acquisitions like TicketFly, and without a clear plan to growing the core internet radio business, Pandora "may be pursuing a costly and uncertain business plan," writes Meister.

He urges Pandora to hire an outside bank to explore a "value maximization process" that could include a sale.

Here's the important part:

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