"Een schip op het strand is een baken in zee."
Posterous should have taken notes from previous sharing sites/functions: Facebook, Digg, MySpace, Google Buzz, etc.
If you change things drastically, your userbase gets mad, and they seek out alternatives. I tend to see social sharing sites as a home online. You put many of your meaningful items there, and you get comfortable with a certain layout, a given set of functionalities, etc. If someone were to come and redecorate/reorganize your room or house without your permission, you'd probably be a little upset.
Facebook eventually mastered the art of introducing change into its site. Nudging the user to try something new ("X of your friends are using the new layout. Join them!") instead of forcing it on them seems to be a better policy.