A few folks have been asking me what I think about this acquisition. I actually think it makes a lot of sense – relative to what Facebook spent to capture users ($30/user) or what Google spent to get viewers on YouTube ($33/user) Yahood per-user acquisition cost is somewhere between $3.67 to $9.40 / user. It also brings a younger demographic to Yahoo's user base, and brings largely non-overlapping content to the party.
Symbolically, it also seems to me that the acquisition gives Yahoo some much-needed cool credibility and an expansion of its engineering talent. It doesn't hurt that Tumblr is seen as an up and coming growth firm while Yahoo is seen as an also-ran.