I think if Twitter hadn't resolved the downtime issues there could have been a mass exodus to Pownce, but Twitter took care of what they needed to and Pownce lost its opportunity. Even though Pownce had a lot of features that Twitter doesn't (filesharing, etc) I never really thought of Pownce when I needed to send a file, link, or image to friends. I always went straight to twitter and used TinyURL or mediafire.

first mover advantage doesn't matter if the first mover product sucks, this is why Google and Facebook were able to get ahead.

But Twitter didn't suck enough for Pownce to be able to get that core segment of new users who are annoyed and frustrated with the competitor.