Using our API it is possible to add contacts to your account which will then generate the welcome email. If you have a way of having your integration determine when someone allows your Facebook application, then it would be possible to then generate the welcome email. However, I am not very familiar with Facebook applications, so I am not sure if there is a notification to you that someone is now using your app.
If you are able to determine this, what you would then do would be make an HTTPS request to our Contacts Collection URI with correctly formed XML that includes the contacts email address. You can get more information about this process here.
Please note that in order to see the welcome email you must make the OptInSource ACTION_BY_CONTACT instead of ACTION_BY_CUSTOMER.
Benjamin Soder NOC Analyst Constant Contact
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