Facebook shares soared 15% Wednesday on blowout quarterly results -- but the stock lost steam after the company admitted young teens are losing interest in the site.

"We did see a decrease in [teenage] daily users [during the quarter], especially younger teens," Facebook chief financial officer David Ebersman said Wednesday, during the company's third-quarter earnings conference call with analysts. He said Facebook usage among overall U.S. teens was "stable," however.

Previously, Facebook had defended itself against multiple recent studies and articles proposing that teens don't find Facebook cool anymore. Last quarter, CEO Mark Zuckerberg said it "just isn't true" that the company has a problem with the teen market.

Ebersman's admission -- coupled with other bits from Wednesday's call, including the fact that Facebook isn't planning to ramp up the number of ads in users' feeds -- sent Facebook shares slightly lower in after-hours trading.

That was a big disappointment given that Facebook (FB) shares had been up as much as 15% earlier in the evening, after blowing away Wall Street's sales and profit expectations for the third quarter.

Facebook's sales jumped 60% over the year to more than $2 billion. Excluding one-time charges, Facebook earned $621 million -- double the company's profit during the same quarter last year.

Strong mobile results: Perhaps even more pleasing to investors was that Facebook's mobile business in particular came in very strong. Mobile ads now account for 49% of all Facebook ad revenue, up from 41% last quarter and easily beating analysts' expectations.

That's impressively rapid growth, considering that Facebook began serving mobile ads just last summer.

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Before Facebook launched those ads, the company's lack of mobile monetization had been a particular sticking point for investors. Shortly after the company filed for an initial public offering last May, Facebook disclosed that it wasn't making "any meaningful revenue" from its growing pool of mobile users. That kept the stock in the doldrums until the company finally launched mobile ads in August 2012.

Now, Facebook stock is up 123% over the past year.

Ad revenue brought Facebook $1.8 billion in sales for the quarter -- and the average price per ad rose 42% from last year.

The remaining $218 million of Facebook's total sales came from fees that the company collects, including the cut off the top it takes from in-app payments.

Facebook now has 1.2 billion monthly active users overall, and 874 million mobile monthly active users.

On the negative side, Facebook's expenses of $1.3 billion rose 45% over the year. The company attributed the jump to hiring more people and to increased infrastructure costs.

Facebook also said it had $9.3 billion in cash on hand at the end of the quarter.

Meanwhile Facebook has also been working to monetize Instagram, the photo-sharing app it purchased last year. Earlier this month, Instagram announced that it will begin placing ads in some U.S. users' feeds over the next few months.