Then they began to have multiple problems. Email notifications would fail to arrive from time to time. Various over-ajaxed functions, including composing a new post and various administrative pages, stopped working on one or another browser. I had to use Chrome whenever I visited my family blog because things would not render properly in Firefox. Then I had to go back to Firefox because files wouldn't upload properly in Chrome. I suspect the problems had to do with the introduction of the "Spaces" thing, which AFAIK never really took off. I didn't even bother to complain because by that time, I'd stopped Posterous for anything important anyway. Then they were acquired by Twitter, and around the same time, most of the problems I'd been experiencing mysteriously went away.
It's sad to see such a great service slowly killing itself. First they ruined a perfectly working product by slapping half-baked social-networking bullshit on it, and then they got acquired by a company that has little to do with email blogging. On the plus side, they seem to have stopped developed "Spaces" since the acquisition, which probably helped prevent the further introduction of bullshit and unstable features.