Investors should sell Snap shares because a survey shows time spent per user on its app is declining, according to one Wall Street firm.

Cowen reiterated its underperform rating for Snap shares, predicting the social media company will report sales below expectations.

“We trimmed our 2Q18, FY18-FY23 estimates, lowering our revenue / EBITDA forecast on slightly lower Daily Active Users (DAUs) and advertising ARPU,” analyst John Blackledge said in a note to clients Tuesday. “Per our recent ad buyer survey, SNAP was lowest Social platform in key attributes like ROI, data and user targeting.”

Snap shares fell 8 percent Tuesday after the report.

Blackledge lowered his price target to $9 from $10 for Snap shares, representing 36 percent downside to Monday’s close.

The analyst noted his firm’s survey of 2,500 U.S. consumers revealed Snap users averaged 33 minutes per day on its app so far in the second quarter, which is down 7 percent year over year.

As a result, Blackledge lowered his second-quarter daily average user estimate for the company to 194 million users from 196 million. He also reduced his second-quarter sales estimate to $248 million from $262 million, which is 2 percent lower than the Wall Street consensus.

— CNBC’s Michael Bloom contributed to this story.

Disclosure: CNBC parent NBCUniversal is an investor in Snap.