Trying to combat fake traffic on your own can be a difficult proposition, but there are some best practices for manual bot mitigation. First of all, it's important to consider your targeting parameters, and to keep them as narrow as possible to limit exposure to fake traffic. Likewise, consider limiting ad runtimes only to hours you know your buyers will be active during. Monitoring your site traffic for unusual Facebook referrals will also provide insight. Fraudulent traffic usually takes certain patterns–large numbers of clicks with low conversions, rapid clicks from one IP address, etc.-- that can give it away. Once you've identified suspicious traffic, an investigation into packet headers can provide corroborating details. Finally, report your findings to Facebook, and wait on a manual investigation.