At least in the cases of Standard Oil and Bell, you would have needed to define the markets as energy and communication (including the post office) for them to not have had near total control.
You could state the generalized product they sold and their total control was evident.
How is "communication" not potentially as broad as "Social media platform"? Basically every form of digital communication falls under the banner of social media platform these days. (The GP comment cites Reddit, YouTube and SMS—as diverse a set of websites from Facebook (while still being communication-focused) as I could imagine)
It is exactly a valid argument. The points you and the parent comment are making are the very heart of the issue in horizontal merger and monopolization cases: what is the relevant market?
Draw it narrowly (I.e., convince the judge to accept a narrow market definition) and making the case against a merger or in favor of monopolization becomes easy. Convince the judge to draw it broadly and it becomes much harder to stop a merger or to make a monopolization claim.
You are misunderstanding me. Yes the issue is exactly about where to draw the line. The parent's dismissed statements that facebook is a monopoly by saying "you can make every company a monopoly if you make the category narrow enough". My argument is exactly what you are saying, you can not dismiss the statement "facebook is a monopoly" by saying "every company is a monopoly if we make a narrow enough category", you need to actually make arguments (and convince the judge) why the category should be this narrow/broad.
