There is a feedback loop between use and ranking quality so that the more people that use google search, the better the ranking will be. I believe they used to use information about which item in the results people clicked on to revise rankings e.g. if 90% of people on pick on result 7 for the same query, this indicates it should probably be no. 1. Whether they still use this or it "got gamed", I have no idea.

One network effect I am particularly bothered by is that developers are forced to design pages to suit Google's crawler & ranking whims less they be penalised to page 2 oblivion. The more people that use Google, the more important it is to design for Google and the better Google search will be with the side-effect of compromising actual content and the needs of the real audience.

This is why monopolies suck, because in practice, I don't have a choice but do this.