Single seller - Google commands 2/3 of search engine volume [1] and 1/3 of online advertising [2]. Yes, they have competition, but in several verticals they are the only practical choice. Note that this does not make them an abusive monopoly, which is what those characteristics seem to be describing more than a monopoly per se.

[1] http://searchenginewatch.com/article/2289560/Googles-Search-...

[2] http://www.bloomberg.com/news/2013-06-13/google-is-projected...