Nice to see some success with advertising revenue. It always seems like HN and several other places are heavily skewed towards charging end users directly. But that's only one option open (And IMHO, it's not quite as scalable and fun as advertising income). If you look at what business models 'power' the web, it's still majority earning their revenue through advertising. Well done :)
Not quite as scalable as premium? I'd argue that premium is far more scalable than free. Open? Maybe not, but scalable for sure. As for fun, money is money. I'll take it in any form.
So with advertising, you can get 10x the traffic, and not need any more resources. It's truly scalable. With a 'paid for' offering, you're more likely to need to scale up customer support, handling charge backs payment issues refunds etc etc.
My comment about 'fun' was more about the overhead and pain required in managing premium payments. Cancelations, mess ups, people wanting refunds, credit card issues, etc. That's not fun (IMHO).
I've always found making money from advertising ridiculously easy compared to trying to charge people.