I wonder if Bell Telephone is a more apt parallel for FB than Microsoft is. I agree FB is not a monopoly today, so I'm not saying this is the case right now, but if they continue to dominate social networks (both through themselves & their subsidiaries), I could see a day coming when they have an effective monopoly on "online social messaging".

Of course, Bell also had a vertical monopoly in that they owned the physical wires in addition to being the service provider, and I think that was part of what brought them down in the end. Unless FB starts buying ISPs they won't be in the same position.