Evan Spiegel, the CEO of Snachpat. Flickr via JDLasica Fidelity Investments has written down the value of its stake in Snapchat, the popular mobile app that lets users exchange disappearing photos, according to a report in the Financial Times.

The only fund manager to invest in the app, Fidelity valued its shares of Snapchat at $30.72 at the end of June and dropped that valuation to $22.91 by the end of September, a 25% decrease in the third quarter, the FT reports using data from investment research firm Morningstar.

Fidelity's most recent quarterly filings from May and August show that the value of its Snapchat shares didn't change in that time period, but a lower market value in a September 30 document of Fidelity's growth company fund seems to confirm Morningstar's data, assuming that Fidelity didn't sell any of its Snapchat shares during the month of September.

Snapchat's last round of funding reportedly valued it at $15 billion.

This news may add fuel to the growing debate about whether the valuations of private companies in Silicon Valley — many of which make scant or no profit — are too high. Investor Bill Gurley is calling for startups that have raised hundreds of millions of dollars to go public sooner, while Marc Andreessen has said that the public markets are too hard on tech companies.

Snapchat isn't the only startup that Fidelity has marked down recently. Both it and BlackRock recently wrote down the value of their holdings in Dropbox, the data-storage company last valued at $10 billion.

Snapchat is one of the most popular apps among the millennial age group, with 100 million users accessing its app every day and viewing 6 billion videos on the service. But the company has yet to prove that it can build a profitable business on the four-year-old service.

Over the last year, Snapchat has continually tweaked its revenue model.

Since ramping up its advertising efforts, the company is reportedly on track to generate $100 million in revenue on an annualized basis, after generating $3 million revenue on $128 million in losses in 2014. But some advertisers feel that Snapchat doesn't offer them enough targeting options or analytics, according to FT.

The company makes money by charging brands to sponsor videos or photos in its Live Stories section, or create their own custom photo filters called "Lenses."

Fidelity declined to comment on this report. Business Insider also asked Snapchat for comment.