Meta points out that it has been here before.
Instagram's Stories feature took a while to get advertisers signed up but is now a big earner.
Meta is monetising Reels more aggressively and expects it to stop losing money around the end of this year.
But the firm acknowledges that it will be a long time before Reels is as profitable as the old news feed.
"We know it took us several years to bring the gap close between Stories and Feed ads," Susan Li, Meta's chief financial officer, said on an earnings call last month.
"And we expect that this will take longer for Reels."
Some wonder if the gap will in fact ever be closed.
Even mature video-apps cannot keep up with the old social networks when it comes to monetising their users' time.
YouTube, which has been around for 18 years, makes less than half as much money per user-hour as Facebook or Instagram, estimates Bernstein.
In China, where short-form video took off a few years before it did in the West, short-video ads last year monetised at only about 15% the rate of ads on local e-commerce apps.
For one thing, the ad load in video is inescapably lower than on a news feed of text and images.
Watch a five-minute YouTube clip and you might see three ads; scroll Instagram for five minutes and you could see dozens.
Watching video also seems to put consumers in a more passive mood than scrolling a feed of friends' updates, making them less likely to click through to buy.
Booking 1,000 impressions for a video ad on Instagram Reels costs about half as much as 1,000 impressions for an ad on Instagram's news feed, reports Tinuiti, a big marketing agency, implying that advertisers see Reels ads as less likely to generate clicks.
Auctions for video ads are less competitive than those for static ones, because many advertisers have yet to create ads in video format.
Big advertisers prize video ads (and report record engagement on TikTok, where products have gone viral with the hashtag #TikTokmademebuyit).
But the long tail of small businesses from which social networks have made their billions find video spots tricky to produce.
Just over 40% of Meta's 10m or so advertisers use Reels ads, the company says.
Getting the remaining 60% to create video commercials may be made easier by artificial intelligence.
One senior executive imagines a near future in which a small retailer can create a bespoke video ad using only voice commands.
Until that moment arrives, half the long tail is lopped off.
Short-video apps are also hampered by weaker targeting.
For audiences, part of the appeal of TikTok and its many imitators is that users need do no more than watch, and swipe when they get bored.
The algorithm uses this to learn what kinds of videos--and therefore ads--they like.
But this guesswork is no substitute for the hard personal data harvested by the previous generation of social networks, which persuaded users to fill in a lengthy profile including everything from their education to their marital status.
The upshot is that many advertisers still treat short-form video as a place for loosely targeted so-called brand advertising,
to raise general awareness of their product, rather than the hyper-personalised (and more valuable) direct-response ads that old-school social networks specialise in.
Here, at least, TikTok's imitators have an advantage over TikTok itself.
Using a trove of data built up over a decade and a half, when there were few rules against tracking users' activity across the wider web, Meta already knows a lot about many of the users watching its videos and can make well-informed guesses about the rest.
If a new, unknown user watches the same videos as a group who are known to be rich female graduates with children, say, it is a good bet that the new user has the same profile.
TikTok says it has made big investments in its direct-response ads, including new tools for measuring their effectiveness.
But it still has catching up to do.
"Meta are leveraging their history," says Mark Shmulik of Bernstein.
Social apps will not be the only losers in this new, trickier ad environment.
"All advertising is about what the next-best alternative is," says Brian Wieser of Madison and Wall, an advertising consultancy.
Most advertisers allocate a budget to spend on ads on a particular platform, he says, and "the budget is the budget", regardless of how far it goes.
If social-media advertising becomes less effective across the board, it will be bad news not just for the platforms that sell those ads, but for the advertisers that buy them.