As a (hypothetical) company with an API of my own, I'd be willing to pay to become a supported channel. This may be unwise initially -- once ifttt's third-party API is ready they'll want as many services as possible on board to strengthen the value proposition -- but perhaps there are scenarios where it makes sense. That is, charge the companies benefitting from increased traffic and awareness rather than the individuals using the system.

User onboarding for their associated services could be a solid revenue stream. Also charging for promoting different services.

Charge to use it?

::GASPPPPPPP::

And have it easily replaced by whomever will run it cheaper. What does Instapaper cost? $1/month?

There are plenty of examples (Apple being the first one that comes to mind) of companies who charge a ton (and make a ton of money) for a product that is available elsewhere for cheaper.

Price isn't always everything in the consumers mind.

What Apple charges for, and what developers/engineers will pay for, are drastically different.

How many people have downloaded Angry Birds vs how many people have paid for Tasker on the Android platform?