He posted a rate of posts (as you mentioned), rounded down, for the "last hour".
When I read stuff like this that covers fields that I know (or blog posts I happen to have read) it really makes me wonder when I'm reading sites covering topics outside of by domain of knowledge, be they Engadget or the Economist.
With more than 300 million monthly unique visitors and 120,000 signups every day, Tumblr is one of the fastest-growing media networks in the world. Tumblr sees 900 posts per second (!) and 24 billion minutes spent on site each month.
http://yhoo.client.shareholder.com/ReleaseDetail.cfm?&Re...
120,000 new accounts every day is amazing. While I don't know how many accounts made up those 72,000 posts over that hour, I can imagine it being easily and quickly wiped out by new posts. 900 posts per second is 3.24 million posts per hour. 72,000 is 2.22%. If that number keeps growing and the 900 posts per second number drops, then this is interesting news. Until then, it's just posturing by WordPress (or perhaps just a cheap publicity stunt).
Again, context is needed. The exact word they used was "signup." Are they referring to new users, or new blogs created by the same users? I recall someone saying that one of the upsides of Tumblr is how easy it is to just create a new blog when you have a random idea for one.
Also, 900 posts/second isn't hard to achieve if most users are using the site to post a pic or a short message.
And yes, a big question is how many of those new accounts are new users verse existing users and spam.