Nice summary, as someone who has both implemented and used APIs of others for as long as I can recall, you offer people out there some solid info.
One additional point I think you may want to touch on is regarding usernames and emails (you did touch on duplicate emails briefly).
Say you are currently sending your API post to 5 different sites. Let's say the username is in use already at three of them. This doesn't mean they already had an account, it could just be they are trying a very generic or common username. How do you proceed?
Some dating sites will append digits or characters to the username, to create one that is unique. This is great as you do not lose out on creating that account.
Other sites simply report back the error that username is already taken. Do you then report an error back to your user to choose another, even though the username was available on your site as well as some of the ones you successfully posted data to? No, that wouldn't maintain a very smooth process. You could do your own appending of characters in an effort to successfully find one that is available... this isn't perfect as it may cause delays as you make the API call potentially multiple times until you succeed.
Another option is just to skip creating an account at those sites. However, while the easiest thing to do, you are leaving money behind.
The same situation can apply for the email address, however it's a more severe problem as one cannot just be modified to succeed; the email must be valid if you have any hopes to convert the member.
For those facing this problem, what I would do, is create an email alias at a domain you control. Pre-arrange this with the dating site in question beforehand, so you do not get blocked for fraud. If my domain where I use the API posts is called hotdates.com and user john888 creates an account with an email address already in use, I would dynamically create the mail alias
[email protected] and forward this to his actual submitted email address.
That comes with its own set of issues, such as making sure you can still get the email through (often, large dating sites will have more resources at their disposal than the average affiliate when it comes to email delivery rate). That is another story alltogether though and best left for another Education Series article
An alternative may be to offer your users an internal mailbox on your site, where they could login and access the mail from multiple dating sites. This would likely not convert well, however, unless your users are used to coming to your site often to check a message box.
Hope that adds some addition food for thought to Choker's article. Enjoy!