91% of internet users is about 3 billion people. They control a platform that affects more people than any single nation state (2-3x larger than the largest nation state and larger than the top 3 countries combined). For instance, changes in PageRank (which is an unaccountable and unauditable algorithm) affect every user of their site in terms of what links they see, what articles they read, and by extension what they think.
You might argue it's the fault of users for not being informed, but I think that if a company has a significant impact on the lives of 3 billion people (which again, is a larger influence than any single government on this planet -- and nation states have constitutions and laws specifically to ensure that they serve the people and are accountable) then it has reached the point where it either needs to be broken up or be regulated. I don't care which, I just think that this cowboy mentality (that software is somehow special and lives in a world where regulation is always an unreasonable viewpoint to have) has to stop at some point.
Regarding the term monopoly, there are different views on what precisely the term means. Google does have anti-competitive practices which you might argue make them act as a monopoly. You can argue they have a monopoly on internet searches because whenever they change PageRank in a way that negatively impacts some users (or when they make changes to GMail's spam filter so that it starts blocking valid emails) there isn't a rush to a different service because there is no way to co-ordinate such a rush. Instead you have other secondary industries like SEO which exist purely to try to keep websites reachable from Google. The fact they have the power to manipulate how the majority of sites operate clearly means they have significant (and in my view monopolistic) control over the market.
They also have a market share that is actually incomprehensible. Microsoft was indicted under anti-trust laws because of Internet Explorer being bundled with Windows. I think Google acts as much more of a monopoly than Microsoft did in the 90s, and "good guy Google" really is an outdated mental model for a company with that much influence over people's lives.
