The first thing you should do is shout your favorite expletives while imagining the horrible tortures you would like to inflict on those who just killed your project. That's a great way to relieve stress and vent frustration.
After you calmed down, your next step would be to write an apology to your customers where you explain calmly and professionally that your service will unfortunately be no longer available in the future because Facebook does no longer allow that kind of access to their service, that there is nothing you or they can do about it, that they should send any complaints about this to Facebook directly and that you would be happy to have them again as customers for your next product which will be announced soon. Then you take your project offline and do a project retrospective.
When everything is wrapped up, you would take a long walk and come up with a business case for a new, completely unrelated project. This time, think carefully if it is really worth the risk to invest time and money into a product which depends heavily on a service provided by someone else who gives you no guarantee whatsoever that their API won't change or disappear without prior notice. Especially not when it is provided by a company like Facebook, which has a reputation for doing whatever they want without considering the interests of 3rd parties or even their own users.