Pinky swear, Marissa? Yes, that was in fact the sub-headline in the official press release yesterday morning when Yahoo CEO Marissa Mayer announced that it has reached deep – oh so very deep – into its pockets for $1.1 billion (CA$H MONEY) to acquire Tumblr and its 100 million+ blogs. The deal will make Tumblr founder and (high school dropout) CEO David Karp, 26, a multimillionaire.

Tumblr is a community of creators and Yahoo needs Tumblr to remain pure, and to keep the core users happy. Mayer has promised to let Tumblr operate independently. That means it won’t be painting the platform purple and adding links to all of Yahoo’s media properties. Karp will stay at Yahoo for four years at least and retain a lot of control over the service, much in the same way Instagram CEO Kevin Systrom does at Facebook.

Here are a few initial thoughts on the acquisition:

– According to Quantcast, Tumblr had 184 million unique visitors, and 12.1 billion pageviews last month. While Tumblr started as a desktop-based service, its mobile offering has ramped up quickly in the last few years. ComScore says that a quarter of the service’s U.S. visitors now come from mobile devices. While revenue has been minimal, Tumblr hasn’t been focused on revenue. They’ve been focused on users and product, which is just what Yahoo needs most.

– Mayer said that the company hopes to bring some of the exciting content on Tumblr to Yahoo. How that will be done is unclear, but you could see how Yahoo might highlight some popular or partner Tumblr posts on Yahoo’s homepage ($$$). This move undoubtedly enhances the offerings Yahoo can deliver to advertisers.

– We’re all shifting our attention to mobile devices, and we’ve adopted the “stream” as our preferred method of content discovery and consumption. Tumblr was built as an activity stream, so stream-based ads makes sense for Yahoo’s display ad business. Yahoo actually rolled out “Stream Ads” on its own site earlier this month and has been shifting its home page and other content sections to a stream like interface. That stream doesn’t work so well with standard display. But it’s great for native units, which Tumblr conveniently features.

Whenever a scrappy startup sells out to an established player, users go up in arms and battle cries go out. All in all, I think this is a win for Yahoo – as long as they don’t screw it up. Hopefully in the end, it’s a win for users.