This is a very good analysis, and I would like to add some historical information to put it in context. Originally, link analysis was based on a simple observa

This is a very good analysis, and I would like to add some historical information to put it in context. Originally, link analysis was based on a simple observation: people were maintaining curated lists of bookmarks in their personal webpages, because before search engines it was the only way to remember relevant webpages; I'm sure that most people remember that every personal site had a "my bookmarks" webpage. Therefore, every link counted as a "vote" from that person to a page. Altavista exploited this by ranking by the number of inlinks. It wasn't long until people figured out that this is very easy to game, so someone came up with the idea of "transitive influence": a site is influent if another influent site votes for it. Algorithms such as PageRank and HITS solved the spam problem. However, with the evolution of the web, the meaning of links changed. For example, most links are automatically generated by CMSs. Also, there is more and more ephemeral information on the web, and for ephemeral information as soon as you have enough links to it to evaluate its quality, the information is already old and irrelevant. Luckily, if you are the most used search engine, you have another important popularity signal, which is given by your users: the number of clicks to a page for a given query. In fact, in today's search engines I would say that the influence of link analysis is smaller and smaller in the overall ranking, and probably done mostly at the domain level rather than for each single page. However, with the advent of social networks, a large amount of clicks doesn't come from search engines anymore, but from social "shares". Which means that the search engine can not observe them anymore, losing a precious signal of popularity (together with "Like"s and "+1"s). I'm not sure if this is what Google is after with Google+, but most probably the click and share data on Google+ affects (or will affect) the search rankings.