I feel like having ads on hubski would severely undermine the experience the hubski team wants us to have -- ads would make hubski look more like a conventional social aggregator, i.e. reddit, i.e. easily digestible content that panders to the lowest common denominator, instead of the awesome website with quality posts and discussions that go on in hubski. Not to say that it wouldn't work, it's just that I've noticed that the interface is very much so designed in order to encourage thoughtfullness and I feel like it ads would detract a lot from it.
That said, hubski needs money. I commend you for giving a lot of alternative funding models, but I'm interested in hearing your thoughs on the model currently being tested.
Edit: In my mind the NPR model would be the best alternative (listed here) to hubski's the model that's being tested now. A close second would be the value added model, which is pretty much the NPR model with benefits for people who donate (thus encouraging more donations).