Though it's still growing in some other parts of the world, usage for several popular social media services is now under pressure in the U.S. and Europe.

To an extent, these declines are being offset by greater usage for other social media services. However, figuring out just how much usage is shifting from one type of service to another is tricky. And as this transition unfolds, the traffic drops that some popular services are seeing are having a revenue impact.

On Tuesday, Facebook (FB) - Get Report reported that daily active users (DAUs) for its core service and Messenger rose 10% annually in Q3 to 1.49 billion. However, in North America, which accounted for 47% of Facebook's Q3 revenue, DAUs were flat sequentially and annually at 185 million. And in Europe, which accounted for 24% of revenue, DAUs rose by less than 2% annually to 278 million, while dropping by 1 million sequentially.

On Facebook's earnings call, Mark Zuckerberg admitted that core Facebook "may be close to saturated in developed countries," even as it still sees decent growth in developing nations that produce less revenue per user. He also called total usage of the core Facebook app "generally stable," which given the growth seen in emerging markets would point to lower usage in more mature regions.

Twitter (TWTR) - Get Report , meanwhile, reported its global monthly active users (MAUs) fell by 4 million annually to 326 million, and that its U.S. MAUs fell by 2 million to 67 million. Global DAUs rose by 9% (no specific DAU figure is given), which represents a slight slowdown from Q2's 11% growth.

Snap (SNAP) - Get Report , for its part, reported its global DAUs were down by 2 million sequentially, and up by 8 million annually, to 186 million. DAUs fell by 1 million sequentially in North America to 79 million, and by 2 million in Europe to 59 million.

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Admittedly, there are some company-specific and short-term factors that are playing roles here. Facebook noted its European user counts have been hurt by an accounting change and GDPR data-sharing regulations. Twitter has been struggling for a while to appeal to a broader set of consumers in the U.S. and elsewhere. And Snap has been hurt by competition from Facebook's Instagram, challenges in winning over older consumers and a redesign gone wrong.

It's also worth mentioning that Instagram, which has remained a bright spot for Facebook amid its recent challenges, reported in June that it had topped 1 billion MAUs. That's up from 800 million as of Sep. 2017 and 500 million as of June 2016. The Instagram Stories service, which launched in mid-2016, was said to have over 400 million DAUs.

And though his comments about core Facebook usage were measured, Zuckerberg was more upbeat when talking about Facebook Messenger and WhatsApp. He noted over 100 billion messages are being sent each day across Facebook's services, and declared that users "share more photos, videos and links on WhatsApp and Messenger than they do on social networks." WhatsApp's stories service, known as Status, had 450 million DAUs as of May.

Nonetheless, there are too many numbers pointing in the same direction to think that there isn't a sea change afoot at least among some consumers in developed markets with high social media penetration rates. In particular, a lot of consumers seem to be paring back how much time they spend idly browsing social media feeds, as well as how much they choose to share to these feeds.

It's worth remembering here that for all of its recent popularity, the social media feed is a pretty new form of entertainment. The concept of spending large amounts of time each day scrolling through a feed to see what comments, photos and videos others are choosing to share in real-time only took off this decade, as did the concept of constantly sharing what's happening in one's life, or what one happens to be thinking about, to hundreds of others. Or the idea of "liking" what others share, and hoping what one shares is liked in turn.

Facebook and Twitter feed usage are undoubtedly facing headwinds. And even on Instagram, feed activity appears to have taken a hit as Stories activity has taken off.

Among some meaningful percentage of consumers in developed markets, there now seems to be a retrenchment going on. By and large, this doesn't mean that these consumers are no longer viewing feeds or sharing content to them; Facebook's feed in particular is still viewed by legions of consumers as a valuable utility for seeing what friends and family are up to. But -- in a world awash with good digital entertainment options -- a lot of consumers are rethinking how much of their free time they devote to feed viewing and sharing, even as some of their friends and relatives still avidly view and share feed content.

Facebook, which has been pretty candid about how feeds are no longer driving its social sharing growth, insists this is nothing to worry about. The company's stance is that private messaging -- in some ways a more natural extension of older forms of communication than feeds -- and Stories growth will more than offset any feed declines.

However, there are still some unknowns here. While Stories have some unique selling points relative to feeds -- the content is immersive and mobile-friendly, it usually disappears after 24 hours and there's no need to worry about likes or public comments -- they could ultimately be susceptible to the same kind of user fatigue that's now hitting social feeds. As is the case with feeds, some consumers are clearly a lot more enthusiastic about avidly consuming and/or sharing Stories content than others.

And though there's little reason to doubt that app-based messaging will remain very popular for a long time, monetizing this activity is a little more challenging than monetizing feed or Stories viewing, given that consumers could recoil at the sight of seeing ads within private message threads. While Facebook has begun showing ads to some users on Messenger's home tab and recently launched a service that (in certain situations) charges businesses to reach WhatsApp users, it has understandably avoided showing ads in the middle of Messenger and WhatsApp conversations.

In the U.S., Facebook also has to contend with the fact that -- as Zuckerberg noted on Tuesday's earnings call -- Apple's (AAPL) - Get Report iMessage remains very popular among iOS users in the U.S.. Given the high average income of iPhone users, many iMessage users are undoubtedly quite valuable to advertisers.

All of this spells a tricky -- though not impossible -- transition for Facebook as feed activity slumps in its most lucrative markets. While Facebook's Stories products have a lot of untapped revenue potential, it's worth keeping a close eye on how their usage trends after two years of meteoric growth. And while its messaging apps are unlikely to see giant usage declines, getting the formula right for monetization might take a while.

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