1. We display the message that you chose for FB as a placeholder text, but always force the user to type in their own message. This is because the https://developers.facebook.com/policy/ states that You must not pre-fill any of the fields associated with the following products, unless the user manually generated the content earlier in the workflow: Stream stories (user_message parameter for Facebook.streamPublish and FB.Connect.streamPublish, and message parameter for stream.publish), Photos (caption), Videos (description), Notes (title and content), Links (comment), and Jabber/XMPP.
2. There's a reason the characters are limited at 100. We stick space character then a link at the end of each message. This link will be about 20 characters long (http://mrk.to/xyz). But then, twitter will run it through their own "shortener (http://t.co/abc)" which can make the link even longer as it has a fairly long path.
Here's a real twitter "shortened url": http://t.co/AatPY3Pg. This weighs in at 21 characters, so 22 with a space in front of it, or 23 if it's https.
We will try to increase the limit to 110 or so, but may not be able to do much more than that.