tl;dr: world monopolies are organically created at the age of the internet nevertheless their lifespan may be shorter than before.
In his book, Zero To One, Peter Thiel has developped an interesting view about the positive aspects of monopolies. Creative monopolies, as he named them, are providing an incredible value to society. Future monopolies is what drives investors to finance innovation, present monopolies is what is able to drive companies away of focusing only on profit.
Though monopolies have evolved compared to the past. We are more and more seeing obvious worldwide monopolies, their scale have changed. They usually are driven by superior products, their societal value creation have changed. They disappear at a new rythm and in a new way, in that sense they are weaker. Let's develop this third point which is widely a consequence of the first two.
Today's world is not flat, let's just say that it is flatter than it used to be. Once you are set for a developed country market, usually going on another is not as tough as it used to be. If your product is providing a superior value somewhere you may be able to reproduce it somewhere else.
Peter Thiel's hypothesis is that you need your product to be at least ten times superior to the competition.
To follow on that, it's interesting to notice that providing a free alternative is not the best way to kill a monopoly. Microsoft Office did suffer the Google apps / LibreOffice / OpenOffice alternative for quite a long time. Adobe Photoshop did not get killed by GIMP and plenty of 3D modelling software are sold when Blender is providing a state of the art free solution. Most free products that actually replaced others did so by a superior experience. If Linux has become a leader on the server market, it was not because of its price. Maybe reliability, security, package management system and superior scripting were a 10 times improvement.
The best way to kill a monopoly seems to be disruptive innovation. The monopoly dies (or shrink) with the business it relies on. At first glance, the innovation that you barely ranked among your competitors, happens to be on the same market as you are. In the end, the best definition of your market is the need you solve, not the technology you apply. Maybe, train manufacturers should be carefully looking at self driving cars after all.
Some actors are moving ahead of schedule, to keep in touch with the need, no matter how good their technology was at some point.
Email, discussion websites, chat services, social networks, messaging apps: communication / socialization. I do really love this example because I experienced it. At a point, I was using online discussion websites, but after some years ICQ / MSN arrived on the market, a way for me to discuss with my friends, rather than only strangers. Focusing on vanity metrics, nothing would have looked bad for my favorite discussion website, my email provider and my desktop chat service. All of them had one registered and active user... The amount of usage though changed a lot. And yes ... I did not know IRC at that time.
One day, a team at Google released what was before a small extra service from our ISPs, a superior, full featured & free email service, Gmail. One day that team pressed a button, and suddenly, all my contacts where available for chatting. My average usage of MSN dropped dramatically, but it was still strong among some of my contacts. Especially the younger ones for which an email address was just a necessary evil to access other online services. What did really killed MSN was Facebook, because there was a melting pot of all generations. All the people you talked with on MSN were there too. Now messaging apps seem to change the game again.
Google is one of the company that has been playing that game very well. There corporate mission was "organize the world information", it would bet that this translate into a need that they are addressing: "information access". Under this view, their whole strategy makes perfect sense: search, email, mobile operating systems, social networks, TV extension systems. Only Google cars don't fit in that landscape.
If we don't think too much about Google Plus, Google has nicely followed the digital uses. First it was about finding things on the internet, then it became about discussing. Social networks are the paragon of the idea that "communication" and "information access" join at some point. Computers looked a bit more than TV than they used to, and we suddenly switched to receiving content rather than looking for it. Then there has been an intense move towards smartphones that now makes plain sense, smartphones are your windows to the world and moving early was the best way to not have to find a ten times improvement later (Chrome OS is not a market leader right now for that reason). Google Now is probably a nice try to replace what social networks are currently best at doing, maybe a machine can push you better content than your friends ever have, that remain to be proven.
Google is trying very hard to stay on top, it's not a monopoly that has been touched by stasis. It's a living monopoly. Even though advertising is still a major part of their revenue, what would this revenue be if it had not been present on your mobile, in your Desktop as your browser and leveraging your social informations?
It knows one thing, that the day it will fail too much for too long, nothing will happen at first. Like it happened before, people are keeping their good old computer, no matter how incrementally better is their Nokia N70. Until the 10 times improvement hit (like real smartphones and tablets), thanks to globalization, software and internet, the inertia that was serving you 6 months before could now be killing you, at a global scale.