I tried to figure out for ages why neither summary or content get transmitted to Digg (which happily accepts a summary using the "bodytext" variable). I thought I did something wrong on my end until I noticed that these fields do get used for emails.
Can you use the summary and/or content variable for social networking sites where applicable? When a site is bookmarked for the first time, the submitter can usually fill these in. Until that functionality is in I'll keep using my own individual digg and stumbleupon links as they're more useful, but I'd love to use share-it at one point because I like the idea of a single link.