I wonder what makes you relate the two. Both are great off-site optimization techniques but totally different at that!
Classified Ads - Much like newspaper classifieds, it's a form of advertising where small impressive ads placed on special classified – ad websites to grasp the attention of viewers. It helps in pulling geo-targeted traffic.
Social bookmarking - It is act of submitting and tagging links on a public (bookmarking) website like Digg, Reddit etc.. with the intent of promoting and sharing information. The technique exposes your content across a wide audience, brings you more traffic and gets you the benefit of backlinks (if do-follow).