Normally, as an app developer, knowing that 100,000 people are using your app is fantastic news. But for Twitter app developers, not so much. Twitter has a policy in place that only allows 100,000 users per third-party Twitter app, and it appears Fenix has hit that number.
In a post on Twitter (naturally), Fenix told one unlucky user the app had run out of tokens. Shortly thereafter, the app was pulled from Google Play.
Where Fenix goes from here is a mystery for now, but it may not be rising from the ashes anytime soon. If you have previously used Fenix, you will still be able to use the app using your old token. New users, sorry.