Before the big social media sites really took off, social bookmarking sites were a great way for users to collect and share content. Some social bookmarking sites, like Reddit and StumbleUpon, still get thousands of users a day and could be great source of traffic for your website provided you categorize your content correctly. But there are dozens, if not hundreds, of old social bookmarking sites that have fallen by the wayside in recent years. These sites might still allow you to post content but don’t provide any real value to your website other than a quick and easy link.
Who knows—in a few years we might not even really be able to call in “link building” because to build something means to actively create it. And if you are actively creating something how can it be natural? From what I’ve seen from Google in the last year or so they really want sites to earn those inbound links with great content, an active social media presence, and more organic online activity.