The social network announced the breaking change in a post on its developer blog:
When we announced the migration from Credits to local currency payments last June, we also enabled developers to receive real-time updates to help reduce latency for their apps and provide asynchronous payment confirmation. We are now requiring developers to subscribe to and honor real-time updates to ensure order fulfillment and receipt of disputes from all payers by May 13, 2014.
We want to provide the best payments experience possible for people on Facebook, and part of that is ensuring that they are able to receive the in-app objects they paid for and that they are receiving timely responses to their disputes. Please visit our documentation on real-time updates to learn more and our documentation on disputes to update us on status of handling disputes.