As the quarterly earnings reporting season gears up, analysts will be watching to see whether social media titans Facebook Inc. and Twitter Inc. can hold their respective market advantages as newer challengers rise to prominence.

Facebook remains the king of the social media space by size and revenue, but analysts note that the company has acknowledged its red-hot growth trajectory may start to cool in 2017. Meanwhile, barring signs of a turnaround, Twitter's struggles with user engagement and advertising revenue could renew M&A speculation surrounding the company.

Facebook has enjoyed year-over-year revenue growth of more than 50% since late 2015. The company reported total revenue of $7.01 billion for the third quarter of 2016, representing nearly 56% year-over-year growth.

"That's just not sustainable," Scott Kessler, an analyst at CFRA, said in an interview. "When [Facebook] reported last, they made it clear that they did not expect to be able to maintain the pace of growth in 2017 that they have delivered in 2016."

For the fourth quarter of 2016, analysts expect the company to report revenue of $8.51 billion, representing about 46% year-over-year growth, according to consensus estimate data from S&P Capital IQ. Facebook is scheduled to report its fourth-quarter 2016 earnings on Feb. 1.

Additionally, some analysts have raised concerns that the growing popularity of Snapchat could erode Facebook's engagement numbers.

Evercore ISI analyst Ken Sena downgraded his 2017 year-over-year advertising revenue growth estimates for Facebook to 36% from 38% in a December 2016 report, citing an internal survey about Snapchat's share of millennial users. Speculation that parent Snap Inc., which has reportedly been valued at as much as $25 billion, could be nearing an IPO have prompted growing interest in the company's financials among investors and tech-industry watchers.

Many observers also have noted that Facebook-owned photo-sharing site Instagram introduced several features borrowed from Snapchat in 2016, mostly notably Instagram Stories, which disappear after 24 hours. Instagram head of product Kevin Weil recently acknowledged the similarity, telling technology news outlet Recode that the feature was "built on a format Snapchat invented … [and] we believe that format will be universal."

However, "from an advertising standpoint, our checks suggest that Snapchat is not having a noticeable impact at all, suggesting to us that its impact is still very modest and only on the supply side, if at all, and not on the demand side," said Evercore's Sena in the December 2016 report. He maintained his "Buy" rating for Facebook. Sena declined to comment further when contacted by S&P Global Market Intelligence.

Kessler said that while Facebook's future monthly active user growth could be "incremental," he believes the company has other potential areas of expansion, such as increasing engagement using Facebook Live videos.

Turning to Twitter, which is set to announce its fourth-quarter 2016 earnings on Feb. 9, advertising revenue and efforts to monetize its platform have been a long-running concern.

In July 2016, for example, the company's second quarter 2016 results failed to meet analysts' estimates, prompting then-COO Adam Bain to defend the company's "premium" advertising prices.

"There are some advertisers that just look at price," he said at the time. "There are more sophisticated advertisers that look at value and price to value exchange. We do think that while we have a premium, that premium is justified."

Ford Equity Research issued a "hold" recommendation for Twitter on Jan. 20, citing a "neutral" trend in earnings per share over the past five quarters.

"While recent estimates for the company have been mixed, TWTR has posted better than expected results. ... Share price changes over the past year indicate that TWTR will perform in line with the market over the near term," the research firm said.

Twitter's consensus EPS estimates for the fourth quarter of 2016 are 12 cents on a normalized basis and a loss of 15 cents on a GAAP basis, according to S&P Capital IQ.

CFRA's Kessler said that while Twitter has been successful in monetizing user engagement on its platform, repeated executive turnover has had a significant impact on the company's prospects.

"Twitter is absolutely a 'show me' story; they need to show people that they can deliver revenue growth. ... We've kind of seen some hint of that in the last quarter … but they also need to sustain revenue growth," he said.

"I think if there's disappointment in the quarter, questions are going to continue to mount about the future of [co-founder and CEO Jack] Dorsey and the future of the company. I think you can definitely see sharks circling to see what can be made of Twitter in other contexts," including potential M&A, Kessler added.