For many companies that start out as a free model. The moment you start to look for monetizing options is hard. Not only is the selection of the right model troublesome, you also need to find the right balance. Push too hard and even your loyal customers run away, aim for too little and you end up not making money.
Finding the right pricing is a constant challenge.
You can create focus groups and survey it, try A/B tests and what quite a few do in the end:
Make custom pricings and force interactions with salespeople.
The latter method is seen very often in business SaaS solutions, where you can then perfectly discriminate for the customer. What IFTTT does is different, they did send out towards the existing customer base a newsletter.
Asking THEM to set the price they are willing to pay (with a minimum amount). That way you are not pushing away your existing customer base as they can co-create it with you and feel involved.
On the other side, you generate insights you can use for the future pricing of new customers. Essentially creating a large trial for your paid model.