I had some issues with the bit.ly service of this plugin (invalid login) which made me wonder what was wrong with my username (=my email). But somehow I found the page
https://bitly.com/a/your_api_key which showed both my API key and my username, which was completely different from my mail address. But it workes! So: before using bit.ly, check your username on this page, especially when you log into bit.ly with your twitter account, you should check this.