> Hmm, the danger there is that one is putting a lot of one's eggs into a fickle basket.

The risk with Substack seems much lower though, since you get to keep their email addresses and could export to another system if you become unhappy with Substack's methods in the future.

With FB, you didn't have email addresses, or a way to export your follower list to another platform, so the consequence of the platform misbehaving was much worse (which perhaps made it more likely the platform would misbehave, knowing that there were no escape hatches).