How to merge user accounts on the site / VKontakte / Facebook?

Question

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There is a registration on the site - the user is assigned an ID and all actions come from this ID.
I want to connect authorization from VKontakte and Facebook.
How can I then put together that one user could do actions first from the authorization of the "site", then go from the "contact" and you need to have centralized data ...?
How it's done?
Thanks

Answer 1

In every social networks have the right to request email during authorization.
Here by email and make a binding. You can even not bother the user explicitly with this.
But a separate page in your personal account with all the associated social. networks must be done
.

Answer 2

Linking accounts from profile settings.
If during authorization through the social network we do not find a user associated with this account, we send it to a simplified registration page (email, password).
Immediately after entering these data, we open the page for linking other social networks. networks.
Situation: the user registered through VKontakte, logged out of the account, after re-logging in he authorized himself through Twitter. On the simplified registration page, I entered the same email and password. In this case, we happily let him in (he also logged in), not forgetting to link this account to his account.
There are a lot of conditions and opportunities to get around the chronic stupidity of the user.
Try to put yourself in their place, you can imagine a lot of things.