Shares of Snap  (SNAP)  on Friday continued to fall below $20 per share as yet another analyst initiated coverage of the stock with a bearish outlook on its future in the crowded social media market. 

Mizuho analyst Neil Doshi slapped a neutral rating on Snap and questioned whether it is "here to stay" or if it will pull a disappearing act similar to its vanishing chat app. Millennials are hooked on using Snap every day, but that's not enough to make it an "Instagram killer" or a "YouTube killer," Doshi said, who set a $20 price target on Snap. 

Snap was tumbling 2.3% to $19.43 on Friday morning. 

The firm surveyed more than 1,000 Snap users to evaluate their activity on the app and found that about 64% of respondents said they hardly ever click on an ad in Snapchat, while only 6% said they find ads on Snapchat to be interesting or relevant. 

"We think this could be due to a weakness in the relevance in the ads on the app," Doshi explained. "...Snapchat will need to demonstrate that they can provide higher quality ads with better targeting in order to increase [average revenue per user] without diminishing engagement."

Like Doshi, who drew comparisons between Snap and Twitter (TWTR) , MoffettNathanson analyst Michael Nathanson said Snap's stock could follow in the footsteps of its struggling social media rival, as its "valuation bubble" may soon pop. Nathanson on Thursday initiated coverage of the stock with a Sell rating and a $15 price target. 

Nathanson said in a research note that Snap's high engagement, "innovative spirit" and revenue outlook are fine, but there are a number of concerns that can't be overlooked, such as its daily active user growth. Investors have also been pushing back against Snap's unprecedented non-voting share structure, saying the move will likely be met with lawsuits in the coming weeks. 

"Anyone reading Snap's S-1 filing could see, clear as day, the platform is already experiencing a disturbing slowdown in sequential DAU additions," Nathanson explained. "Over the last two quarters, Snapchat's rate of sequential net additions has slowed dramatically." 

This metric, along with the fact that Snap has yet to turn a profit, make it look "a lot" more like Twitter than Facebook (FB) . Nathanson said Snap's current $20-billion plus market cap a "valuation bubble." 

Cantor Fitzgerald analyst Youssef Squali made similar observations in a Wednesday note, saying Snap's "unproven" business model makes it seem less like a $20 billion-plus public company and more like a start-up that's still figuring itself out. Squali yesterday initiated coverage of the stock with an Underweight rating and an $18 price target. 

None of the other eight Wall Street analysts who currently cover Snap have buy ratings on the stock, although one analyst did upgrade his rating on Wednesday. 

Squali's heaviest criticism of Snap was aimed at its "rich" valuation numbers, which upon review, just "don't add up."

"The valuation looks stretched to us both based on the future cash flows of the business and when compared to peers like Facebook and Alphabet's (GOOGL) Google at IPO," Squali wrote, estimating that Snap trades at an enterprise-value-to-revenue (EV/R) multiple of 28.6x. Analysts compare a company's enterprise value to its revenue as a means of determining whether a stock is priced well. 

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By comparison, Squali noted that Facebook trades at an EV/R multiple of about 9.6x, while its peer group, which includes Twitter, Alphabet and Yelp (YELP) , trades at an average EV/R multiple of approximately 5.5x.

CFRA Research analyst Scott Kessler, who upgraded Snap's stock to Hold from Sell on Wednesday, said some of the concerns may be "excessively negative" because Snap could demonstrate "considerable growth" as it focuses on user monetization. Kessler added that Snap's stock has fallen below the firm's price target of $22, which is part of what warranted the upgrade.

He still views increased competition and a lack of corporate governance as factors that could hurt Snap in the long term, however. 

Snap is also struggling to prove to advertisers that its business is a meaningful investment beyond just an "experimental" revenue opportunity, Squali said. This presents major issues for Snap when considering that advertising revenue made up 96% of its top line in 2016 and could decline further due to revenue sharing with ad partners. 

Additionally, early data suggests that Facebook and Google may be stealing ad dollars away from Snap, according to a survey of about 1,600 marketers, conducted by RBC Capital Markets and Ad Age. Marketers in the survey said they're seeing a lower return on investment from Snapchat due to increased competition from Instagram, a lack of key metrics used to measure ad performance and decreases in user engagement. 

"We believe that Snap is still a start-up to a large degree," Squali explained. "The company has much work ahead to prove to these marketers that their ROI on the SNAP platform is not only positive, but equal or better than other platforms which compete for these ad dollars, namely Facebook/Instagram, YouTube and Twitter to name a few."

Facebook and Google are expected to continue to dominate the digital advertising market in 2017 with a controlling share of 19.7% and 40.7%, respectively, according to eMarketer. 

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