First, Facebook isn't the only place to share revenge porn, so they aren't really solving the problem, only reducing it slightly. Second, security by obscurity

First, Facebook isn't the only place to share revenge porn, so they aren't really solving the problem, only reducing it slightly. Second, security by obscurity is not a good thing. Not running the algorithm locally, not to mention producing the source to their algorithm is not going to help in the long run. Third, regardless of whether the source of the algorithm is known or not, it would be a simple task to upload a test image of oneself, and then attempt to circumvent the algorithm using dummy accounts. Fourth, if someone abuses the service by uploading the hashes of a benign images, it can be reported at that time. No need to pre-emptively guard against this.