In the recent Webmaster Help video, Google's head of Search Spam, Matt Cutts explains how Google separates popularity from authority. The question tossed was:
“As Google continues to add social signals to the algorithm, how do you separate simple popularity from true authority?”
Emphasizing strictly on the latter half of the question, Matt said that popularity has nothing to do with authority. Just because a website is popular doesn't mean that it will be linked to as an authority. Explaining the concept with an example, Matt said that although porn sites are very popular, people generally tend to avoid linking to those sites. On the other hand, government or organization sites do not generate a lot of traffic but are often more authoritative than porn sites.
Matt revealed that even in earlier days when page ranks first showed up to the users, they were described as “a measure of the popularity of websites.” This frustrated Google because this was just not the case. Matt explained that while popularity denotes where people are going on the web, page rank is a measure of reputation determined by inbound links.
In order to distinct popularity from authority and determine whether a site is the appropriate match for a requested query or not, Google looks at how topical the anchor text is on the inbound links. If the links pointing to your site use the same keywords and phrases repetitively, Google will view you as an authority on that particular topic.
Matt also revealed that Google is planning to introduce some new changes to its algorithm to determine on which topic a particular site has the authority. Google will deviate from the common phenomena of giving ranking preference to popular sites and search for clues within the site which indicates that the site is an authority on a particular topic.Matt Cutts Explains How Google Separates Popularity from Authority!,