Is there any way to find all comments on external websites connected with a Facebook account? Let's say I want to find all my Facebook activity on external pages based on profile URL; is there any way to do this?
Unfortunately not. Facebook does not allow the end user who uses Facebook connect to see all of your information across the web that uses the Facebook login. Here's the Facebook page to see how Facebook connect works. Facebook does allow you to see in your account settings which apps/pages you have authorized to connect to your Facebook account, as well as revoke that access.
It may help to read the Facebook comments plugin site for developers to see how it works on the site's end.