Does this mean posterous isn't doing well?
Anecdote: for our Quantter blog, we chose Posterous first. But then Marco Arment (busy) advisor of us, just told us: sthg like "Posterous?? those guys keep on trashing us (ie Tumblr)". While 1st impression preferring Posterous product, the switch to Tumblr was easy because of Posterous aggressive attempt to "poach" another startup users. That was not good karma.
Huh, what? What evidence is there that Posterous has lost any battle against Tumblr? Just because you chose Tumblr after being advised by one of the founders of Tumblr doesn't mean Posterous has "lost a battle"...
Posterous is prolific, but it's not omnipresent. And I doubt it will ever become omnipresent, because it doesn't offer enough to differentiate itself from Tumblr and because it's nowhere near sexy enough to capture the vast audience that only cares about things being sexy.
The suggestion that Garry Tan is leaving because Posterous isn't working out is unsubstantiated, and that's what I'm arguing about. This is like pointing at Facebook when they were much smaller than MySpace, showing that such-and-such person just left, and saying it's because Facebook has lost the battle. It hasn't. The battle is still raging, and who will win is definitely up for grabs.