If users are logged into Facebook, clicking the Hello FB button opens a comment modal. If users are not logged in, they get the [appropriate] log in form for FB. However, after clicking 'log in' the sharing count is increased and no comment form is offered to the user. The "Like" is registered on the FB activity history. Google+ and Twitter both have areas for users to log in & add comments at the same time.
Once users are logged in, they can click to share again & at that time get the comment form- so they do have that option.
Confirmed this behavior repros on other FB share widgets.
As we can't affect behavior of the FB widget, can we close this bug out?