Monopoly is reached, when there is a lack of economic competition to produce the good or service and a lack of viable substitute goods.

If the service facebook provides is access and contact between people, any substitute (ie, proof that there is no monopoly) would have to be near the size of facebook to qualify. How many other viable substitutes are there that reach 30% of the population? Any at 20%, 10%, 5%, 1%?

face it, facebook has a monopoly right now in regard to a service proving access and contact between people.

uh.. skype? twitter? reddit? aim? gmail? google plus?

You're ignoring the root cause of this discussions. Which service that you're listing has a 'real name' policy?

  Skype: Pseudonym (this one)
  Twitter: Pseudonym (the same)
  Reddit: Pseudonym (the same)
  AIM: Uhm.. Nope. Guess allows pseudonyms? Not sure
  GMail: No restrictions either
  G+: Same as FB, hopefully the next target on the list of this group.

So - most of your list aren't playing that 'give us your real name and be prepared to hand over your ID to prove that fact' game with their users. Which is what the discussion is about. Of the services that do this, FB clearly is the leader. Or in a monopoly-esque situation.

You're shifting the issue. We're not debating which large service is requiring real names (btw, G+ has the same requirement), we're debating whether or not Facebook is a monopoly. By your logic, I could make the argument for any service being a monopoly by being specific about the type of service they are supposedly monopolizing. For instance, Twitter has a monopoly on microblogging services that use birds in their marketing.

It's interesting to exchange these retorts with you.

We really have completely different views and probably (and 50% is certainly on me) trouble understanding each other.

Microsoft was (is!) a monopoly. Although you can buy OS X, get Linux, FreeBSD, OpenBSD, NetBSD, Nexenta, SmartOS, Solaris, Haiku-OS or whatever floats your boat. [1]

Facebook is a monopoly, even if there are products/services available that provide something kind-of, sort-of similar.

1: Apologies for missing someone's favorite alternative OS in the list

I think the point is that it's very difficult to define exactly what a monopoly is. There's a lot of disagreement on that topic.

Comparing Skype with Facebook is comparing apples with oranges.

Google plus and Twitter are closer, but a Twitter handle is not drop in replacement for Facebook group. That just leaves Google plus.

And Google plus has around 0.5% of the German population as users.

0.5% vs 30%... not a replacement exactly. almost like saying that AOL search is a replacement for Google search. However, I think the ad networks do not agree with that.