Snap shares closed down 5 percent Thursday after a Wall Street investment firm found that ad buyers ranked the company lowest relative to other social media networks.

Cowen lowered its rating for the photo messenger's shares to underperform from market perform, predicting a 30 percent decline in stock price over the next year.

Analyst John Blackledge cut his 2018 revenue expectations to $1.1 billion from $1.3 billion and warned clients that fourth-quarter earnings are likely to disappoint, citing a Cowen survey of social media advertisers. That survey asked 50 senior U.S. ad buyers to rank the major social networks across various characteristics.

"On the whole, Snap was the lowest rated relative to other Social networks, given low relative marks on return on investment; targeting; and data, analytics and measurement," wrote Blackledge in a note published Thursday. "Ninety-six percent of ad buyers would prefer to advertise on Instagram Stories vs. Snap Ads (4 percent), but underscoring the opportunity ahead, more than 50 percent has yet to purchase Instagram Stories ads or Snap Ads."

Snap did not immediately respond to CNBC's request for comment for this story.

Snap has had a turbulent history since its IPO last March, plagued by volatility and sliding prices; shares are down 13 percent in the past six months. Blackledge's $11 price target represents 28 percent downside over the next 12 months.

Adding to concerns, Facebook's rival photo application, Instagram, is expected to be the biggest share gainer among any social network per ad buyers, putting pressure on Snap performance in 2018.

All ad buyers who spent cash on Instagram Stories in 2017 expect to repeat that spending, with many others indicating that they'll start spending to the platform in the new year, according to the Cowen research.

But to be sure, the survey did find some reason for optimism for Snap investors.

Snap's critical metric, daily active users (DAU), is expected to increase 16 percent year over year in the fourth quarter and remains a key measure for the company, according to Cowen. The company's recent redesign of the Snapchat application is intended to drive active user growth by simplifying its interface.

"We believe investors are eager to hear a response to the recent app redesign, and in particular whether or not it can jumpstart daily active user growth," wrote Blackledge, adding that Snapchat users are also spending more time on the application.

Those between 18 and 24 years old used the platform 53 minutes per day in the fourth quarter, according to the analyst, leading to an overall uptick in engagement for those already using the app.

And while increased engagement is good news for the company, Blackledge did note that Snap may be running out of room to grow among its most active users.

"We dug a little deeper into the time spent numbers and found that Snap's strong showing was highly influenced by the strength of the younger age cohorts," said the analyst. "This poses an issue as Snap has already reached 70 percent penetration of 13- to 34-year-olds in the U.S., France, the U.K. and Australia. The challenge for SNAP is whether the user base can age up."

Disclosure: CNBC parent NBCUniversal is an investor in Snap.

— CNBC's Michael Bloom contributed to this report.