Marissa Mayer just made her biggest bet to date as Yahoo’s CEO — and if it goes badly, she may not get the chance to make another. A series of recent acquisitions culminated Monday with the announcement that Yahoo is spending roughly one sixth of its cash hoard to buy Tumblr, the popular but under-monetized blogging platform. The deal brings Yahoo the vibrant, youthful social network it has long lacked, but it also sticks the company with an expensive and unproven young business. Meanwhile, the authors of 100 million Tumblr blogs want to know: can this Yahoo acquisition succeed where so many others have failed?

Mayer has been looking to make a significant acquisition since August, when Yahoo reached a deal to sell its stake in Chinese web giant Alibaba for $4.7 billion. She has been on a buying spree from the time she arrived at Yahoo last summer, scooping up 10 mostly smaller companies including Summly, Astrid, and Loki.

With earlier acquisitions, Mayer was interested mostly in acquiring engineering and product-management talent. More recently, it used the acquisition of Summly to improve news summaries in the Yahoo mobile app. And while Mayer’s acquisitions have yet to result in a significant new product, they signaled a newly feisty attitude from a company that many were writing off. Yahoo’s name began popping up in stories about bids for high-profile content sites including Hulu and DailyMotion; the New York Times reported it also talked with Foursquare and question-and-answer site Quora.

With Tumblr, Yahoo is making a straightforward bid for growth — in users, in web traffic, in revenues, and in relevance. All have been in decline in recent years, and a series of CEOs have been unable to reverse the slide. Yahoo, which once dominated the online display advertising market, has been elbowed aside by Google and Facebook.

The Tumblr acquisition brings Yahoo a younger demographic, a thriving social network, significant product and design talent, and a potentially lucrative new home for display advertising. Tumblr reportedly earned $13 million in revenue this year; John Saroff, a former advertising executive at Google, estimates Tumblr could generate $108 million annually based on standard rates.

The question is whether Tumblr will give Yahoo the social lift it has sought for so long, or whether it will devolve into the second coming of Geocities. Before Mayer defected from Google, Yahoo had become notorious for acquiring companies for billions, neglecting them, then ruefully shutting them down. Geocities, Broadcast, and Delicious are among the company’s ill-fated acquisitions; Flickr languished for years before Mayer began investing in it again.

Mayer is looking to quell any worries on that front by allowing Tumblr to continue operating as an independent company and retaining 26-year-old founder David Karp as its leader. All Things D reported that Karp would stay on at Yahoo for four years, in an arrangement not unlike the one Instagram co-founder Kevin Systrom has at Facebook. But where Facebook’s Instagram purchase united two stable leadership teams, Yahoo and Tumblr have both been wildly unstable compounds — Yahoo with its revolving door in the CEO office, and Tumblr with its high turnover in the executive ranks.

Still, while users of social networks can be fickle, Tumblr has proven adept at attracting them: the number of blogs on the network doubled in the last year. The company has invested heavily in its mobile team over the past 12 months, unveiling new or redesigned apps for iOS, Android, and Windows Phone. Mayer repeatedly emphasized on this morning's conference call how excited Yahoo is by Tumblr's mobile audience and metrics. And the blogging network is a steady source of pop-culture phenomena, from Hipster Puppies to STFU Parents; Karp recently said that more than 70 users had gotten book deals from their blogs.

Yahoo’s challenge will take the vast amount of attention Tumblr attracts and make it consistently profitable. Karp was once openly hostile to the idea of advertising on Tumblr, and while it eventually came to users’ dashboards, the company has been cautious in rolling it out. Ads on Tumblr take a different form than traditional display advertising; instead of paying for clicks, marketers use Tumblr to create interactive campaigns designed to spread virally. Similarly styled ads, called "native" because they appear in the same place as regular posts, are the norm on Google, Facebook, Twitter, and other places; Yahoo has yet to deploy native ads of its own. Home Depot and The Great Gatsby are among the major advertisers who have used Tumblr to generate buzz, though it’s unclear what sort of return they’re getting on their investments.

For now, most businesses don’t have the Tumblr presence necessary to build a campaign there. Yahoo’s own struggles with display ads in recent years leave room to doubt whether it can scale up Tumblr’s take on native ads. And Tumblr users have enjoyed a mostly ad-free experience over the past six years, and could balk if Yahoo ramps up ads too much too soon. On this morning's conference call, Mayer insisted that Yahoo's advertising efforts on Tumblr would be tasteful, limited to the central dashboard or users who specifically elected to have ads on their Tumblrs. The change will also be gradual. Yahoo said it doesn't expect to see a material impact on its business from Tumblr until next year at the earliest.

But of course there will be challenges — few billion-dollar deals win universal praise from the outset, and investors have more than earned the right to be skeptical of Yahoo’s big acquisitions. So far, though, they’re backing Mayer’s grand strategy — Yahoo’s stock is up 70 percent since she took over. If Tumblr continues to grow and Yahoo succeeds in turning it into a platform for native advertising, today’s $1.1 billion could someday look like a bargain. But that day is still a long way away.

Additional reporting contributed by Russell Brandom.