I've successfully linked Facebook, twitter, Disqus, and Google+ to my phbBB3 forum. It works and I can use all four to login to the forum.
I've enabled "Social Network Linking" and it works as intended with Facebook and Google+ meaning that if there is already a phpBB3 forum user with firstname.lastname@example.org email and the user has used the same email address on Facebook and Google+, it links.
However, with Disqus and twitter it does not link the user if there is already a phpBB forum user with the same email address. Instead it just creates a new user in the forum with an imaginative email address.
What am I doing from?