I’m not any particular fan of Facebook, but to run with the cancer analogy: normally a cancer consumes and destroys its host. To the best of my knowledge, Facebook’s market valuation has basically only gone up for the last decade. The only (and very recent) bumps for then have been privacy related, which are closer to the core business than to any diversity initiatives they might have. So where, exactly, is the cancer?
To run further with the analogy: it can go years without being diagnosed. Right now Facebook is in the 'I feel fine, I don't need to go to the doctor' phase. By the time the market starts penalizing a company things are usually on the decline, if not in free fall.