I think it was only six months. And first mover advantage means little online. Facebook wasn't the first social network, Twitter wasn't the first microblogging app, etc. Posterous's had a year and a half now and it's stagnating.
The Compete data seems to show Posterous stagnating just shy of 1M uniques for the past few months while Tumblr's curve is stuck "up and to the right," though.