While this is the case (and Facebook and Microsoft are definitely ready to pounce), Yahoo currently has Tumblr negotiations under lock and key, Forbes reports:
A source with knowledge of the Yahoo/Tumblr discussions says they are proceeding rapidly and likely to result in an offer as soon as Yahoo CEO Marissa Mayer can get her board’s approval. She is said to favor an outright acquisition rather than an investment or partnership. As mentioned above, a lockup agreement prevents Tumblr from holding a bake-off with Facebook, Microsoft or anyone else.
Part of Mayer’s courtship has involved working overtime to convince Karp that his fears of being “absorbed into a behemoth” and “raided for talent and traffic” are unfounded. Karp has made it clear that he would like to run Tumblr for a long time. According to my source, he would remain CEO under the outline of the deal being worked out.
Those last two sentences are key.