Even very hardcore libertarians will admit the need to break up monopolies.
Not me. Freedom > Justice.
I'm as hardcore an-cap subscriber as you can get and I'm perfectly fine with monopolies that are established by providing a voluntary service or goods.
The issue comes when the monopolies arise due to market distortions from government-granted privileges.
Food for thought - where would microsoft be without patent and copyright law? And is it truly voluntary when they pull tricks such as telling hardware vendors that customers buying "naked" computers (those without an OS) are criminals?
As for facebook though, in no way is it a monopoly or a threat to anyone's privacy - if you don't want your personal info there then don't provide it, this seems so simple it's stupid.