I thought this was a valid point brought up by Anil Dash on Twitter (http://twitter.com/anildash/status/17380509761):

"Of course Twitpic should allow export, but Posterous calling others "dying platforms" is an unnecessary dick move. Hence, pettiness."

Here's where they used the term "dying platforms": http://blog.posterous.com/make-the-switch-to-posterous