I don't have much luck with the main SB sites these days, only the main few are worth submitting to.
So many SB accounts get deleted, mainly bookmarks are no-follow links, and main bookmark websites are oversaturated, so your bookmark will soon be on page 34567.
In my opinion it is better to submit to less known do-follow websites if you are into backlinks,
or if you are into traffic than to main players like facebook, digg, twitter, delicious, reddit...