As joeld points out one big difference between ThoughtStreams and Pinboard is that Pinboard was effectively a clone of a site people were familiar with but abandoning. In other words, Pinboard didn't really have to worry about people needing to try it first to "get it".
The same is true of App.net. People knew it was basically a Twitter clone that was subscription-based rather than ad-based so it was an easy sell for people looking for an ad-free alternative to Twitter.