Specify custom text for social sharing

I'd like to be able to specify the actual text that gets tweeted or shared. It seems that what gets written in there by default is the blog title and URL. But there is no opportunity to add any descriptive text, hash tags, etc.

Most users will not know, care, or bother to add anything more compelling than that, so the sharing will not have much impact.

We really need the ability to customize each post depending on which social network it is going to. I want to use more than 140 characters on Linkedin, for example.

Otherwise, a really great and exciting addition. But this is really crucial to make it really effective.