I think the value you provide isn't the notification, but the surrounding data.

Consider that my app may not have deep integration into Facebook, or Twitter, or whatever. I just have a simple signup page that accepts the user's name, email, and a password. Where your app could shine is if I could send the information I do have on the user to your API endpoint, and it takes care of going out and figuring out who this person is.

"Hey, the email address this person used to sign up for your service is also tied to a Twitter account with 1.5m followers" or "A person with the same name as this user is a tech writer for Gizmodo" are things that could be very important, but that most individual devs or small shops aren't going to have the resources to discover for all of their users.