They have a paid ratio of around 6% (around 35M subscribers, 2M paid.) Some correlating of past subscription and revenue figures suggests each subscribed user nets Substack $11 revenue. So perhaps 130 million total subscriptions to hit 100M revenue.

If that's compared to a mediocre social network like Twitter, which supposedly can show ~$45/year/user of ads, if Substack can get a quarter of its non-paid subscribers to view ads that are a quarter as effective (smaller economies of scale, weak ad partner etc.), they would still quadruple their total top line revenue.

And it's all upside from there because they can get better at selling ads at scale and they can grow unpaid followers by diluting their brand once they're confident it'll pay off.

That, unfortunately, explains the VC case to me. I'd much rather them be a smaller company that focuses on taking real money.