MySpace in 2007 had 80% of social network traffic, turns out it could not do what it wanted. Advanced tech economies generally consist of monopolists replacing each other, not bazaar economies.
It may still be possible to beat a monopoly by being extremely good and aggressive right on time when your competitor is napping. e.g. Firefox and Chrome vs IE. If IE team hadn't been dismantled and added tabs and other UI features, worked on performance and security improvement, we may have been using IE 25.
Less risk than if they weren't monopolies, which is the entire point.
Not necessarily, especially if the switching cost is too high. For example, if Microsoft does something monopolistic, which they had done, people aren't going to switch to Linux. Therefore the government had to step in.
Shareholder democracy does not count.
In fact the government is nothing else but a very large version of this, a big monopolist who solves a collective action problem.