Recent app download data for Snap suggests a slowdown in the growth of daily active users, a key metric for Snapchat's parent, Nomura Instinet analyst Anthony DiClemente said in a note Wednesday.

DiClemente cited data from SensorTower, a firm that tracks app analytics, which showed Snapchat's year-over-year downloads have dropped 22 percent in the first two months of the second quarter.

"By comparison, Instagram downloads have demonstrated YoY growth, suggesting that competitive pressures may be intensifying for Snap, challenging the platform's ability to attract and retain new users," he said. DiClemente, who has a reduce rating and a $14 price target on the stock, also lowered his full-year sales and profit estimates for 2017 and 2018.

Snap did not immediately return CNBC.com's request for comment.

Shares of Snapchat's parent hit session lows Wednesday shortly after DiClemente's note was released. The stock traded about 3.7 percent lower just before the close in New York.

It's been a rough and volatile ride for Snap investors ever since it went public on March 2. Snap has fallen nearly 20 percent since then as worries about user growth, competition and monetization have mounted.

Snap shares since IPO

Source: FactSet

User retention and growth have been big concerns for several analysts on the Street regarding Snap, including DiClemente, especially with social media giant Facebook putting more pressure on the firm.

Facebook-owned WhatsApp released its own version of stories — a feature that lets users share videos and photos for up to 24 hours — in February, less than a year after Instagram, which is also owned by Facebook, launched its own version of stories. Snapchat was the first social media platform to come out with such a feature.

Last month, Facebook CEO Mark Zuckerberg told analysts that Instagram Stories, had 200 million daily active users, while WhatsApp's version had more than 175 million. By comparison, Snap had 161 million daily active users at the end of last year.

"Our overall takeaway from these datapoints is that Instagram's strategy of replicating key aspects of Snap's use case is bearing fruit, limiting Snap's ability to attract new users," DiClemente said.

Disclosure: NBC Universal, the parent company of CNBC, is an investor in Snap.

Watch: Wall Street gets nervous about Snap