Disagreed. They are de-facto mandatory to use. A former university I was at, for instance, published some required information on facebook. Saying "well, they should not do that" is as correct as it is useless and futile in practice.
I believe the information was visible to either all logged-in users or just all visitors, but that still requires facebook to serve the page to me.
You can say this about anything, though. "My university requires us to use Blackboard, therefore it's a de facto monopoly." "My work email account is through Gmail, therefore it's a de facto monopoly." They're still not. If another party requires you to use a service for some reason, that's between you and the other party.
Let me make this simpler, since there are a lot of comments like yours:
1) If I want to exchange redlined documents with lawyers, I need Microsoft Word. I cannot run a successful business which deals with law firms without Microsoft Word. Most businesses need to deal with law firms. If Microsoft shuts me out of Word, I cannot have a business.
2) If I want to promote my local business, I need to be on social media platforms which my likely customers use.
3) The same goes for niches. If I'm supporting K-12 writing teachers, I need to support Google Docs.
It's not a question of alternatives, best-in-class, or anything else. It's pure network effects. If a platform is >50% dominant in my market, I need to support it, or I'm out-of-business. No one will switch from Twitter to Mastodon or Parler for the sake of doing business with one small business. They'll go next door.
Once a firm has that level of market power, I think it ought to be regulated, both for the same reasons and in the same ways as railways were in the days of Standard Oil.
These companies can literally just kill a small business if they chose to. That's not healthy.
> 1) If I want to exchange redlined documents with lawyers, I need Microsoft Word. I cannot run a successful business which deals with law firms without Microsoft Word. Most businesses need to deal with law firms. If Microsoft shuts me out of Word, I cannot have a business.
I find your reasoning here disingenuous. I have been running a business for almost 2 decades, dealing with law firms and everything and I haven't used Word since I was in high school.
Took a consulting gig with RedHat once. RedHat asked for a document. I gave them a LibreOffice .odt doc (that I wrote on Fedora). They rejected that doc due to inability to access it. I sent them a LibreOffice exported .docx file and they again rejected it due to formatting issues. At that point they specifically requested I use Word and send them a Word document.
Microsoft Word makes the world go round. Sure I can use Wordpad and export a docx file, but no tables, no special effects, etc
Google Docs, gitlab or bitbucket, and as for social network there's plenty out there.
None of those are real monopolies. They _might_ be best in class, but there's no rule that says you must be allowed to use the best in class service.