Yikes-- I misread the graph. The post is adjusted. Sincere apologies to folks for the incorrect analysis on my part.
The point stands though. This is a race without a finish line. With two runners circling the track, it's more about who cramps up when. Tumblr was at ~7m uniques per month at the stage that Posterous is currently at (~2.5m uniques). Certainly ahead, but not quite the blowout the RWW is reporting.