I'm not seeing that at all. I'm testing while being logged into Facebook in another tab, using Chrome on a Mac. I see the below when I click the Send button:
Can you let us know how you're testing, so we can try to recreate. Also, please try from other browsers, different facebook users, etc, to see if you can narrow down what makes it happen. We've never heard of the Send button opening a new window, so are a bit unsure what could be causing it.
Thanks,
Alex