Looks cool. I think this is more suited to Instagram than Facebook. Even though Instagram is wildly popular, you still don't get to see as much third party tools like they have for twitter.
Also one question :
Would giving permission to post result in the app making postings without my knowledge?
Been using streaks for few days and so far not impressed. App hardly does anything beyond sending reminders. And even reminders are not particularly motivating. They just say "Did you do this today". Uninstalling.