I don't think there is a monopoly in search, look we have duckduckgo for instance.
Since it's their search and their google+ why aren't they allowed to use one as metrics for the other?
Not only does google have twice the market share of anyone else, they likely have more than twice the number of advertisers as well. This means they are making more money on each visitor. That's why Yahoo and Ask both had to outsource their ads to Google, even though they had large market shares. Blekko has cited this as a huge reason they are having trouble competing.
Creating a real search engine (not a meta-search like Duck Duck Go) from scratch is very expensive. Blekko started out with 700 servers. Until you get to a very high scale, the server costs are prohibitive.