A picture taken on May 20, 2013 in Paris, shows logos of the brands Tumblr and Yahoo! on the screen of tablets. Yahoo! bought today the popular blogging platform Tumblr for $1.1 billion. (FRED DUFOUR/AFP/GETTY IMAGES)

What does Yahoo see in Tumblr? In a word: youth.

That much was clear from Yahoo chief executive Marissa Mayer’s remarks Monday about her company’s decision to buy the blogging community for $1.1 billion. Mayer talked about how Yahoo’s older and Tumblr’s younger audiences fit together like “Africa and South America.”

Put another way: The blogging site’s main users are in the 18-25 age group, according to the analytics firm Quantcast — meaning that the youngest members of Tumblr’s core audience were born in 1995, the same year Yahoo was founded.

(As for what Tumblr sees in Yahoo, you can probably come up with about 1.1 billion reasons.)

That hyper-connected, hyper-engaged audience is exactly who Yahoo wants to attract and keep on its side. You can already see Yahoo straining to appeal to all the hip kids on Tumblr with the wording of its release — a straight-forward promise not to “screw up” the community. The Web giant is being careful — very careful — to say that Tumblr will remain pretty much as it always has been despite having new, purple overlords.

Tumblr, Mayer said, won’t get any Yahoo branding and will operate and grow in a way that’s completely independent of the Yahoo brand. In her remarks, she drew comparisons to how her former employer Google has handled the acquisition of YouTube — the two brands are clearly related, but Google has largely allowed YouTube to keep its community’s identity.

Judging by the posts tagged “Yahoo” on Tumblr right now (that we can quote from, anyway), a lot of people aren’t buying it. Those posts highlight three things that Yahoo absolutely cannot screw up (read: change) if it wants to keep Tumblr users happy: culture, content and advertisements.

Chief among users’ fears is that the growth Mayer touted in her conference call will kill Tumblr’s intimacy. Though Tumblr has gained 100 million users since it was founded in 2007, the site still doesn’t have quite the mainstream recognition of Facebook, Twitter or even Pinterest. With Yahoo backing, that’s likely to change.

“The thing I fear with Yahoo’s acquisition of Tumblr is that its not gonna be as close-knit as it was before,” wrote one Tumblr user.

“I don’t know who I hate more: Tumblr for not being themselves’ and literally selling out. Or Yahoo! For changing something that has absolutely nothing wrong with it,” wrote another.

There’s also concern that Yahoo will tighten Tumblr’s relatively lax content standards — for example, there’s quite a bit of adult content on Tumblr that advertisers may not want alongside their brands. That content aside, Tumblr users are worried they’ll lose the freedom to post their opinions — no matter how unpopular — openly on the site.

In her call, however, Mayer said she’s not too worried about eliminating what one analyst referred to as content that isn’t “brand-safe” from the site.

The diversity of Tumblr’s content, she said, is what makes Tumblr engaging. She said that Yahoo simply needs to have “good tools for targeting” advertisements.

Which brings us to the third major concern among Tumblr users that Yahoo’s acquisition will see Tumblr completely overrun with advertisements.

Advertising is certainly something that Yahoo is interested in doing on the site, which only recently warmed to the idea of in-line sponsored posts. Mayer was pretty clear about this on the call as well, saying that Yahoo and Tumblr interact differently with brands, but can have complementary strategies.

Plenty of companies and organizations use Tumblr specifically to build a branded home on the site that appeals to younger users. As The Washington Post’s Rachel Weiner noted last month, even the White House is using Tumblr to reach younger users, posting video, photos of Bo Obama and animated GIFs. Branded homes combined with Yahoo’s ad network, Mayer said, could help boost revenue.

Mayer also said there will be more advertising on Tumblr, pumping up what’s already on the site, and perhaps working with bloggers who want ads on their Web site. With permission, of course, she added.

It might seem like a no-brainer that a relatively ad-free Tumblr couldn’t last forever, but the companies will have to be very thoughtful about how they roll out ads to keep users from fleeing en masse.

In fact, the first wave of emigration may already have happened. According to a post on the personal blog of Matt Mullenweg, a founding developer of WordPress, the blogging site has already seen some spikes that he believes could be tied to the Yahoo announcement.

On Sunday, when tech blog All Things Digital broke the news that Yahoo’s board had approved the deal, Mullenweg reports that WordPress saw “over 72,000” posts imported over an hour from Tumblr — a dramatic increase, he noted, from the 400-600 hourly posts the company normally sees.

Related stories:

Yahoo acquires Tumblr in $1.1 billion deal

Should Yahoo buy Tumblr?

White House using Tumblr, lecturing us on ‘GIFs’

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