Having a competitor be 4 million uniques ahead of you when you're 2 years into a consumer internet race? I dunno. Wordpress.com, by comparison, has about 160m uniques per month (per quantcast). Tumblr and Posterous are still in "small potatoes" country-- it's anybody's game.
4M vs 400K is a pretty wide margin. You also tried to compare growth rates which didn't exist. Finally you're also are showing a graph with Tumblr's growth climbing and postereous's growth pretty flat. If anything you just reinforced the point that Tumblr is leaving postereous in the dust.
