Ergo facebook is not a monopoly.
It's ironic that I would have to define monopoly here - I personally have a great deal of distaste for facebook's actions. In my opinion they engage in highly deceptive business practices. Their users are unaware of the damage to their privacy stemming from facebook's actions.
But unethical or distasteful practices do not imply a monopoly. Google does not have a monopoly either - if you prefer not to use their services, use Bing or Baidu or DuckDuckGo ...
I understand that facebook and google embed javascript on other pages. So download the requestpolicy firefox addon. Or disable javascript. And cookies. I think you can see my point: you, the customer, can spend your money and your bandwidth (= money) other places if you prefer.
That's not what a monopoly is. By that definition, no company has ever been a monopoly.
Many people and businesses have no choice but to use Windows. It's nearly impossible to buy a computer without Windows, especially if you're picky about configurations. Even if the choice exists (Apple), switching involves considerable costs: buying all new hardware, moving to different application software, retraining personel, dealing will legacy and proprietary file formats.
In contrast, moving to a different search engine is easy and free. There are no barriers to switch.
Downmodding us adds almost nothing to the discussion.