Google probably achieved in making the Black Hat SEO's realize the importance of natural links. One after the other it penalized or devalued most of the link building strategies that were generating spam in the search results. Spam is something which Google fights 24*7 and updates like Penguin is a direct response from Google to combat web spam.
Recently Google penalized link networks like Buzznea, Ghost Rank 2.0, Backlinks.com, Sape Links etc. All these networks established relationships between advertisers and site owners in order to provide text links by paying bloggers and site owners money in return for a keyword rich do follow text link. Buying and selling Page Rank is an activity that is against Google's guidelines. These link networks made a lot of money and had remain in existence for years together before they were spotted and taken action upon.
Link Building Strategies that Have Become Stale
Link building using article marketing has long been a useful link building tactic for link builders but rise in spammy articles forced Google to penalize sites having a higher percentage of low quality links from article content farms.
The situation that stands now is that this link building technique is of little to no value now.
Guest Posting for Backlinks
Guest Posting if used as a lone strategy for building backlinks can activate Google's spam filters. Guest posting should not be done for gaining backlinks. Matt Cutts recently confirmed this in a blog post.
Most of the comments have been marked as no follow and leaving comments just for the sake of getting backlinks will increase spam on the web and nothing else.
Bookmarking on popular sites was a good strategy for building links in older times but the current semantic search scenario demands more than just backlinks. Hence, bookmarking should not be used as a link building strategy.
Bye-Bye Link Building, Welcome Link Earning
As said in this post, all forms of link building strategy is considered unnatural by Google. This is absoutely TRUE.
If you don't believe me today, you will regret later.
There is no strategy which can be said as "White Hat Link Building Strategy" because if you are building links for yourself then that link is of little value. What remains now is link earning and citation earning. So, how do we do that?
Please read the below posts in order to learn more about link earning and citation building.