This was only illegal because Microsoft was deemed a monopoly. One could certainly argue that Facebook has a monopoly, but such an argument would definitely be much more difficult than the one against Microsoft.
FB might be a bigger fish than we might be aware of.
Thoughts?
It's a 400 billion dollar company and, as the GP noted, most people's default web experience.
Those are the only ones.
I think we're aware of the size of the Facebook fish, but you might not be aware of the size of Microsoft at its peak.
Something like 75% of adults in the United States have a Facebook account[1], but at the height of their dominance Microsoft operating systems ran on ~98% of computers [2]. 75% is a lot, but it's probably not a monopoly - especially because Facebook does not make the devices or the OS on the devices. Lots of people use Facebook, but nothing is keeping them using it.
Being popular is not illegal.
[1] http://www.usatoday.com/story/tech/2015/01/09/pew-survey-soc... [2] https://arstechnica.com/features/2005/12/total-share/8/
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.
They tried (are trying?) becoming that in poor countries with their 'free' internet initiative.
So a company might have close to 75% market share in an industry that matters and they would be a lot easier to prosecute vs the company w/ 100% market (a fleshlight, or other trivial niche thing)
More importantly, you could buy a computer and phone and live without Facebook (or Instagram. My life would be poorer without Whatsapp though).
However, looked at another way, Facebook is now too important to be a private company. It has access to so much data about so many people that the potential for abuse is huge. I wouldn't want to see them become a government owned entity, as the potential for abuse would be just as great (maybe greater) while innovation would inevitably slow down.
But I would like to see a regulator that sits on Facebook's board, can review its source code and order changes.
My dream would be for Facebook to be broken up and turned into a protocol. But it seems that ship has sailed.