It doesn't matter who is at fault, ultimately. If you pay for a service, you expect that service to be provided. If you pay to use an app for 30 days, and suddenly you can only use it for 15 days, through no fault of your own, the company should refund you half of what you paid.

Regarding fault: Twitterific absolutely shares responsibility here. They built an app and service that depended on the whims of a third-party service that they could not guarantee perpetual access to. They should have had a plan for what to do if they lost API access... and not a plan that begs their paying customers for charity.