Snap  (SNAP - Get Report) may generate less money than predicted from its primary revenue source in 2017, according to research firm eMarketer, doing little to quiet analysts' and investors' concerns about the young social media company's path to profitability. 

Shares of Snap slid 2.8% to $20.51 on Tuesday, hitting a post-debut closing low since the Snapchat parent company's blockbuster initial public offering earlier this month, when the stock opened trading at $24 a share. Shares are still trading above the initial offering price of $17, however. 

Of the eight analysts who currently cover Snap, none have given it a Buy rating. Aside from Snap's slowing user growth, the company's reliance on advertising revenue is also a cause for concern.  

Snap's advertising revenue in 2017 could be $30 million less than previously estimated, eMarketer wrote on Tuesday, meaning that analysts' fears aren't likely to go away anytime soon. Advertising accounts for the vast majority of Snap's total revenues, comprising 96% of its top line in 2016. 

eMarketer now predicts Snap will see $770 million in advertising revenue in 2017, down from its prior estimate of $800 million. 

The adjustment in advertising revenue doesn't reflect a material concern about Snap's business, said eMarketer analyst Cathy Boyle. Rather, it reflects a more precise view of how much money Snap shares with publishers, Boyle added. 

Specifically, eMarketer concluded that revenue sharing with advertising partners, which includes the companies who take out vertical video ads and have spots in Snapchat's Discover section, was higher than previously estimated. Snap has partnerships with publishers such as The Washington Post, BuzzFeedCNN and  Vice, with Vice just announcing an agreement on Tuesday to  produce exclusive video content for Snapchat.  

But FBN Securities analyst Shebly Seyrafi said he disagreed with eMarketer's ad revenue estimates, noting that their ad revenue number comes in far below his projections for Snap to generate $1.03 billion in revenue in 2017 (Wall Street's consensus estimates are for $1.04 billion). Given the current percentage of revenue that comes from advertising, that would put his annual ad revenue estimate at about $989 million.

"As I understand it, Snap is trying to move away from revenue share with publishers," Seyrafi explained. "Instead, like with TV, it wants to pay for content and do less [revenue] sharing."

"If it succeeds here, then there would be revenue upside," he added. Per Seyrafi's estimates, Snap's revenue would surge 158% year-over-year in 2017, up from the $404 million it saw in 2016.

Meanwhile, rival  Twitter  (TWTR - Get Report) will see its ad revenue slump by 4.7% this year, the firm estimates, as it suffers from flat user growth year-over-year and other issues. 

By 2019, Snap is likely to surpass Twitter's share of the mobile ad market, the firm said. Twitter's share of mobile ad spending is expected to drop to 2% this year (down from 2.6% in 2016), while Snap will account for 1.3% of the U.S. mobile ad market in 2017. But in just a few years, that share could grow to 2.7%, eMarketer said. 

"Snapchat's ad business, which is made up entirely of mobile display, is still small," the firm added. That said, it could soon eclipse Twitter's mobile ad business, which is worth more than $1 billion. Twitter also sells ads on desktop platforms. 

Several analysts have noted that Snap's fundamentals should be evaluated using a long term view. 

Snap has partnerships with some of the "biggest" brands and publishers, making it "well-positioned" to scale rapidly and grab an increasing share of the $652 billion global advertising market, said analysts at venture capitalist firm Goodwater Capital. 

"Since younger people are spending more time on mobile devices and sharing videos, Snapchat seems to be well-positioned for the growth expected in mobile and video digital advertising," the firm added. 

At the same time, Snap still faces mounting competition from Facebook (FB - Get Report) and  Alphabet (GOOGL - Get Report) , which are estimated to control 24.6% and 32.4%, respectively, of the mobile ad market.