It depends on which bookmarking site you are asking for ? Normally I use the keywords in the title and in the content of the article too, so it eventually shows up in the bookmarking description place. So if you wanna place your keywords at the right place, then make sure you insert them in the title of the article as well as the first paragraph of your article.
Normally when you bookmark a page, then its title comes in your description place. You are free to change, or replace it. Because that is your own property. If you are asking about your bookmarking on your browser!! and if you ask other thing, I could not give answer properly.
try to use a keyword synonym or flip the keywords . likr if ur post is on "linkbait creation" then use the word "create a link bait" while submitting - reason ? If you submit the exact phrase your sites title then the social bookmark link will rank higher in the search results than your main article since they have high PR and authority and that will defeat the purpose of bookmarking .