Neatly executed ! & a nice idea! Few suggestions:

1. Actions seem to lead user across several pages ; IMO, they could be done in a single page (for example, managing an already-added-twitter-account).

2. When I invite someone, you seem to auto-create an ID for their email id. Am not sure if (a) this might end up creating a lot of unused/spam Fatnest accounts, (b) the user (who is invited by email) may not really like it, because he/she might or might not 'actually want' to sign up.