The couple months I was referring to are the last few. The preceding months contradict your thesis.
I think you're mistaking how much license a blog gives you to write about "the situation as I see it today." It's ok to write stuff based on small amounts of data, but the conclusions you're entitled to draw from it should be correspondingly small.
They don't contradict my thesis since Tumblr was always much bigger than Posterous and while they may have had faster growth it was never enough to catch up.
Again -- I may be proven wrong in six months or a year, and if so I admit it publicly right here (in fact, do you want to make a bet?). But I think at this stage the data warrants the trend I extrapolate from it.
