lovelyderriere:

It is worth noting that all that has taken place is an offer.  It still remains up to Tumblr to decide whether to accept. 

While it is difficult to turn down over a billion dollars, we may hope that tumblr will see this offer as a validation of the tremendous potential for increased value in the years to come.  If only they will see it in that way, it would be unwise to sell now.

I expect they will almost certainly take the money and run, but that is not the only possible outcome.