They're not going to block someone they are partnering with Facebook is a business. They'll do whatever is in their business interests. If it is "block and cop

They're not going to block someone they are partnering with Facebook is a business. They'll do whatever is in their business interests. If it is "block and copy such and such app" and they can do it without significant reputational damage (which is really the only protection YC startups will have), they will do it.