Very true, but the examples you cited also involved long-time users abandoning those services. The author was careful to note a plateau, not a downward spiral.
Would I argue against Tumblr eventually hitting a downward spiral (negative growth)? Certainly not. I think that is more likely to happen before it sees no growth (positive or negative).
Additional edit: Mind you, I have no statistical evidence to support these claims. These are purely some thoughts that I have, and, quite frankly, I wouldn't mind being proven wrong.
Would I argue against Tumblr eventually hitting a downward spiral (negative growth)? Certainly not. I think that is more likely to happen before it sees no growth (positive or negative).
Additional edit: Mind you, I have no statistical evidence to support these claims. These are purely some thoughts that I have, and, quite frankly, I wouldn't mind being proven wrong.