I've created a group on Facebook for rationalists and effective altruists to discuss philosophy (currently has about 120 members). To be clear, this isn't a group where the philosophy has to be related to effective altruism or rationality, but instead a group for Rationalists and Effective Altruists to discuss philosophy.

I know it's already possible to post about philosophy here, but people tend to be reluctant to post unless they've invested a huge amount of effort in writing it up, so it's useful to have somewhere else where there's a lower barrier.