Posting comments on blogs with links seemed like an easy way to accumulate authority for your site. As a result, it ushered in an era of blog comment spam (which unfortunately continues to this day).
Bots attack the comments section of WordPress blogs and leave a large number of "click here to buy the magic pill" comments.
Spam comments
Nofollow links
To prevent spam and PR manipulation in comments, Google, in partnership with MSN Search and Yahoo, launched the nofollow tag in 2005. The idea was to restrict the flow of PageRank through some links.
A nofollow link is a backlink that has the rel="nofollow" attribute spain mobile database in its HTML code. This attribute tells search engines not to pass authority through that link.
Generally, there are two main situations in which the nofollow attribute should be used:
1. Untrusted Content
If you don’t want to inadvertently pass PageRank to bad communities on the web, then nofollow is a good option.
This is why most commenting systems (including native WordPress comments or services like Disqus) use nofollow links by default.
2. Paid links and advertisements
Due to widespread link selling, Matt Cutts recommends using the nofollow attribute on paid links . He says: What if a site wants to advertise purely for visitor clickthrough traffic, or has a purchased link? In this case, I would use the rel="nofollow" attribute. The nofollow tag allows a site to add a link that does not count as an editorial vote. Using nofollow is a safe way to purchase links because it is a machine-readable way to specify that a link does not have to be counted as a vote by search engines.