Similar API's are available all over the web. the best seems to be www.pittpatt.com but it's kinda pricey. A big advantage is that PittPatt can be installed locally on a server to provide a secure app. Privacy could be a concern as the images are stored on a CDN.
A big question is what is the confidence score of accurately matching images? The smaller the dataset the better but this was a big concern with the app that i was building. General uses like celebrity sites wont have this concern.