We do meet the FB terms of service as we're using their native button to subscribe and also everyone has to opt-in first.
Good point on the dependency on FB, it's actually a good and a bad thing. The good point is that your subscribers don't have to install new apps and they already have FB Messenger on their phone and probably are logged in inside chrome so the subscription is one-click and friction here is minimal.
Another point is that we do not rely on Facebook only as we also support Slack, Telegram Chrome, FF, Safari and others as a subscription platform so the user can choose which one suits best.