Investors care about ARPU, which is measured by dividing revenue by number of users in a quarter, because it shows how effectively companies monetize their users. These social networks are becoming victims of their own success — the pool of new potential users keeps shrinking.

If user growth starts to go flat or decline for these companies, revenue growth must come from squeezing ever more value from existing users.

Facebook is now entering that stage. It remains the clear leader in ARPU, but investors were spooked last week when the social network announced its number of daily active users in the U.S. and Canada fell to 196 million from 198 million in the third quarter. Facebook said it expects its user base in that region to remain flat or decline in the next quarter, too. Its user base in Europe also remained flat at 305 million daily active users for the third consecutive quarter.

But there may be little space for ARPU improvements in the U.S. and Canada region, as Needham analysts pointed out in a note last week. ARPU there was already a whopping $39.63 last quarter, compared with $12.41 in Europe, $3.67 in Asia-Pacific and $2.22 in the rest of the world.

The key to growth may lie in Facebook’s broader family of apps. On the company’s earnings calls, executives discussed early efforts to integrate messaging apps with ads so that businesses can run ads in Facebook and Instagram and take users to a Messenger or WhatsApp thread. Facebook is also expanding its e-commerce business through Facebook and Instagram, and executives suggested last week ads could come to Instagram Reels, Facebook’s TikTok competitor.

These types of auxiliary services for Facebook are still in the “early innings of monetization,” wrote KeyBanc analysts.