I have built a web application that allows users to login with Google and Facebook through OAuth 2.0. I actually used the SimpleAuth project on Google App Engine. In my database I am storing their OAuth ID, which for Google looks like:
https://www.google.com/accounts/o8/id?id=AItOawnrcuEaKdY_EMesk8FdeFNGP-cCKJbmVf0
and for Facebook looks like:
facebook:1494270173
and I am wondering if I can use the same method on an iOS or Android app. I know that I will need a much different implementation but will that ID be constant even if I am using a Facebook or Google login on mobile?
The Google identifier you show is an OpenID2 identifier, not an OAuth 2 one. OpenID2 will not work on iOS nor Android. Not familiar with SimpleAuth, but if you can configure it to use OAuth 2 or OpenID Connect (which is based on OAuth 2, not to be confused with OpenID2), then using the OAuth 2 protocol on iOS or Android will give you the same identifier for the same user.