You can now edit your comment in facebook. This was known by me by chance.

Edit comment in facebook 1

Edit comment in facebook 2

When you write a comment and press enter, the comment is posted. At that that time if you try to remove the comment, the comment will not removed but you can edit it as shown in figure 2. Edit the comment and wala you are done. Now no need to copy the comment , delete it and make new comment in facebook.

Now you can edit any comment in picture, your status, wall post, or any other activity on facebook.

Note: This feature is only available when you just comment and press enter key and then try to remove it, then when u try to  remove , you can edit.