Although it's rumored that Google looks at how many people click on a site, the more important criterion they are believed to use is the number of other sites that link to a site.

This is claimed to be the reason that sites trying to sell nothing in particular are likely to have "reciprocal links" to other sites similarly trying to sell nothing in particular. Some sites have home pages consisting of nothing but links to "other useless sites like mine" with most of the links going to sites with nothing but links to "other useless sites like mine" at the target pages. Of course it's considered "bad form" to omit a link to any site that links to you(?).

"Social networking" seems to encourage this practice, with the predictable result that soon Google will find only pages of vacuous twits and peeps.

John