You can use social bookmarking for getting the good ranking, but it is for short time, i.e. through the social bookmarking you can get good ranking for short time period but through the forum posting you can get for a long time. So forum posting is better for getting good ranking.
Social bookmarking gives the exposure to your site in the internet world and one can access the bookmarked site from anywhere in the world with just an access to the internet.It is much different from the Bookmarks which a personal computers have.It can only be accessed by the person who owns the system.
Through social bookmarking one can get decent amount of traffic and backlinks at the same time.