For this and many other reasons, I gave up connected accounts a few years ago. I understand that it is convenient, but if someone really wants to register on your forum, they will.
It can also encourage some "bad habits" by users. Since disabling Facebook, I've been contacted by some people who were using it exclusively to sign into their accounts — they didn't know their email addresses or passwords, and since they didn't know their email address, they couldn't recover it. And because their email address wasn't really valid, we were never getting the chance to bring them back to the site with notifications.
Where can I find such stats?
If you're comfortable running queries directly on the database, you can see exactly who is using what.
You can see how many total users are using connected accounts using this query:
SELECT COUNT(*) FROM xf_user_connected_account;
To filter by a specific provider (in this example, Facebook):
SELECT COUNT(*) FROM xf_user_connected_account WHERE provider = 'facebook';