Google most certainly doesn't share user's search history with Facebook. There may be some tracking on the sites themselves, that allows Facebook to detect user

Google most certainly doesn't share user's search history with Facebook. There may be some tracking on the sites themselves, that allows Facebook to detect user's interests. However I don't think it's supposed to use one persons web history to show ads to another user. What might be happening is that they are using some common accounts or a shared browser that leaks the data between them.