Could you give an example of an API that didn't bring new users and one that did?
Assuming that both services have comparable user bases, was the difference caused by demand alignment between the source site and your site, or by poor support on the source site for discovery of your service's integration?
Yes, of course. Being on Facebook app store brings me 10 users/day and Google apps for Businesses brought me 1 user/month before I switched it off.
There should be a website that lists the apis and gives review on them, even if I have to pay for it