I'd like to prepopulate the exact message that is being shared. Is that possible?

I realize that each of the social networks has a slightly different way of deciding what information is going to be shared, so this may not be possible...

Right now, the default message that is prepopulated for Twitter is 33 characters over the limit on one of my ads, because it includes Site Name + Site Title + Post Title + Download Link. I don't really need the site name and site title appearing in the message. But more specifically, I'd like to be able to have a "Desired Message" field when I set up each ad, which would help with Twitter, although I understand it wouldn't affect Facebook Likes. I'm not certain how LinkedIn and Google+ pull information from a page for sharing, but if there is a way to specify a shortened URL to share, there must be a way to specify some other message variables as well. Does that make sense and is it something that could be done?