Both StudiVZ and MySpace are examples of how Facebook isn't a monopoly. So is Twitter and e-mail and Google Plus. And PHPBB and Diaspora.
Facebook has a monopoly on being Facebook. For everything else, there's ample competition. The fact that Facebook is more popular than the competition doesn't make it a monopoly.
Would you consider Windows as a monopoly? There are alternatives.. If your answer is 'No', the EU seems to disagree with you (and might on the FB case, we'll see). If your answer is 'Yes', where's the difference?
Windows is more of a monopoly in the sense that there is much heavier lock-in: It's not trivial to write an application in such a way that it works equally well under Windows and the alternatives. The lock-in to Facebook is much, much lighter.
But more importantly, being a monopoly (however such is defined) is not in itself illegal, and it's not a carte blanche for publicity-hungry politicians to hand-wavily demand to control random details of a company's operations.
What is illegal is monopolistic behaviour, to leverage your dominance in one market to unfairly gain an advantage in another. That is what Microsoft did and was punished for (rather than merely being a monopoly), and what Google might be getting in trouble for.
WHAT? Pardon, but I don't see at all how you could come to such a conclusion. The primary assets of Windows are: ease-of-use, ubiquity, hardware compatibility. But if you have an application you love that works on Windows, chances are (if it's not a game, but even then there are other games and entertainment software) that there is a port or clone for another system (like Linux or Mac OS X — I'm speaking consumer-grade stuff here, business is another matter entirely.)
Facebook on the other hand has these assets: your Mom's on Facebook, your dad's on Facebook, so is your neighbour, your aunt, all your friends, (ex-)class mates, acquaintances, that boy or girl you had a crush on in first grade…
Your social graph is much more difficult to migrate to a different social platform than it is to port an application from one OS to another (or to find it on the Net.) Because it is pretty much impossible. Mass-migrations sort-of happen (like StudiVZ → Facebook a while back in Germany, or Digg → reddit, but there the social cohesion was rather low,) but only in high pressure situations. You can't easily bring about such a situation without the point of origin of said migration fucking up in a major way.
Facebook, Twitter, Instagram, Tumblr — they're all sitting on social gold, and they're keeping it for now.
Monopoly means that there is only a single seller of a given commodity.
Windows was a monopoly in the 1990s and until I guess around when OS X came out - these alternative applications you mention didn't exist yet. Today I wouldn't say Windows is a monopoly, partly because good-enough alternative applications exists, partly because many (most) new, fancy applications are web-based and not limited to IE (as they once were). Today, really, there is really only one reason to run Windows, and it's Office - and that's going away fast, especially as users dawn on the fact that they can communicate without e-mailing Word documents around.
Another important component of Windows monopoly is exclusivity. Today I can trivially run Windows in a VM for that fraction of my workflow that isn't properly supported on Mac or Linux - 10 years ago, the best you could do was dual booting, which was painful and tricky for non-technical users.
Which brings me to Facebook: No, you can't port your social network lock, stock and barrel to a competing service. But you can trivially, with very little friction, maintain your presence on multiple networks - all you need is another tab in your browser, or a client app that supports each. You can even use the APIs of the respective services to integrate them with each other - you can automatically post Tweets to Facebook and vise versa. Plenty of people I know use both Facebook and Twitter in parallel with various degrees of automated integration. That's not the face of a monopoly.
The fact that other people don't want to stop using Facebook does not make Facebook a monopoly. They have strong network effects, yes, but that's not the same.
Facebook has a monopoly on being Facebook. For everything else, there's ample competition. The fact that Facebook is more popular than the competition doesn't make it a monopoly.