I would argue a large part of the problem with essentially a "donate" button is that the flow is all wrong for a service provider, especially one that basically only has a one-time-setup cost for implementation. Consider a likely sequence for someone who goes through the whole process:

  * X visits your site, is mildly intrigued.
  * X notices the donate button, passes it up because you have
    provided no real motivation for them to do so
  * X gets an API key and pokes at it lightly
  * X integrates
  * ... time passes ...
  * X is appreciative of the time saved, and might give you money...
    IF they saw the button right then.
    ELSE more time passes without paying.

I haven't seen filepicker.io's implementation (which has since been taken down, apparently), but did they include a 'donate' button in the API documentation? Did they email people after X time had passed, or Y API calls? The vast majority I've seen never take these steps, they just slap a 'donate' button on the site and wonder why it doesn't make them rich.

If they didn't repeatedly try to poke people, it's out of sight and out of mind. The hallmark of any good service, really. It's unfortunate, but not surprising, and I wish I knew a more reliable way to get money out of people who are appreciative.