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
