I was wondering what happens when a user first logins with the facebook or google or twitter provider and there is already an existing user with that username (which eventually registered throught dnn without those providers) or when there is already an user registered with the same email (most probably in this case the same person trying to access now with the providers) and of course you have both unique emails and unique username enabled.

The merging feature is a good thing to carry on in my opinion BUT of course it cannot be done based on the username but on e-mail address.
It's a good thing because if someone is already registered, it won't be able to login with oauth providers unless he creates another user (or merges it).
Thanks for your kind reply. I discovered DNN about a month ago (because It's only 2 years I switched to .NET) and I'm always more and more addicted to it... I'm just sorry I got to it so late :)