send dialog in modal box?

There's no way to do that with the current Send button as that content is loaded directly from Facebook.

The upcoming release will have a modal popup for Open Graph Actions that can be used as well as a new share type widget. Both of those use different types of popups, and we've made a big effort to make things more responsive with the next release. With that said, there's likely going to be some issues once it's rolled out to all users since every template is different, but we'll gladly help with any responsive issues that may arise.

I hope that helps explain,
Alex