Patreon seems to be a better model, to be honest. It's about nine million dollars a month in total pledge, which meant over one hundred million in revenue per year if it never grows beyond that.
I prefer Flattr's model to Patreon, as it lets me spread donations around without much commitment, e.g. when googling for error messages and finding helpful blog posts. A Flattr<->Patreon bridge might be a nice idea, so creators using one can receive donations from the other.
Unfortunately Flattr's mechanisms for transfering money were overly complicated when I used to use it: to keep down third-party transfer fees, each Flattr account maintained its own balance, and could be manually topped up or withdrawn from in bulk using a relatively obscure payment gateway.
This obscurity and manual intervention discouraged topping up, and made it very easy to forget. I also seem to remember those receiving Flattrs being unable to withdraw anything until they reached a certain minimum balance, which was difficult to achieve for a relatively unknown network of micropayments.
I'd probably start using it again, if I could forget about my "Flattr balance" and just have it automatically top-up by $X from my bank account via some common gateway, whenever there's not enough to cover the end-of-month payment; I could then manage it in the same way as my existing charity donations.
EDIT: The above was based on my experience in Flattr's first few years of existence. I've just logged in and there now seem to be many more payment methods, and an auto-topup option :)
IANAA but generally if they're just processing payments then GAAP revenue would only be the difference between cash in and what they immediately pay out. Think of it in terms of facilitating a transaction and taking a cut of what flows through their hands.