The money quote, literally, on subscription v. ad models for social platforms:
"You’re banking on making more revenue from a small subset of passionate customers willing to pay for the product than you could from showing ads to the whole customer base....It’s risky to rely on keeping consumers paying over long periods of time, but beautiful if it works."
Read that carefully. It's risky only because you might not achieve theoretical maximum revenue through a subscription model. The ad model could bring in more revenue than subscriptions...that's the risk you're taking as a business owner when you choose a subscription model over an ad model.
As most business owners know, some risks are worth taking.