Also, mobile-based photosharing was deemed a "major threat" to Facebook's existence and relevance, whereas Tumblr is a much less mobile-centric and is deemed more of an "asset acquisition" and doesn't carry the urgency that Instagram carried for FB.
Buying Tumblr though? It's not so much of a preventive measure. It would be more for the integration potential and userbase than anything else and I think most people are saying Tumblr lacks potential as a standalone service. However, if it could help augment the Yahoo brand then it could potentially be worth it for Yahoo.
I think there are multiple angles to look at, and we shouldn't focus too much on the price as much as the motivations behind such a deal and what the outcome could potentially be.
Perhaps Yahoo could build services around Tumblr? Or maybe they'd elect to integrate Tumblr into Yahoo itself. Or perhaps use it add to their portfolio of companies to strengthen the Yahoo brand. I could also see a Yahoo ecosystem consisting of a tag-team of Flickr, Tumblr, and their News aggregations. So the question remains; what is their strategic motivation for Tumblr?
Yahoo made it crystal clear that they own Flickr, for example.
Do AOL or Advanced Publication make it clear they own Huffington Post, Tech Crunch, or Reddit? Not really. Sure, it's in black and white on their 'about' pages or via Wikipedia, but they generally had a hands-off approach. Huffpo and Reddit are still growing; Flickr not so much (except in some demographics or countries).
Maybe Yahoo will keep a hands off approach. That also helps create a firewall for Yahoo stockholders who might balk at Tumblr's questionable content.
When you don't have that dynamic, Yahoo! doesn't have to buy Tumblr. Maybe they have to buy something but it doesn't have to be Tumblr and it doesn't have to be now, right this moment.