The 'my colleagues are on twitter by not mastodon' example doesn't feel salient to me. The best example would probably be MMORPG's. Should people who play WoW have access to people who play FFIV? I guess the question would be is social networking more a communications utility like a cell phone or e-mail, or is it more of a recreational activity. Personally I've never felt like not being on facebook took access away from my friends the way not having a phone would. Oddly no one talks about LinkedIn being a monopoly which feels like a more substantive example, since not being on LinkedIn may impact job opportunities. But not being on facebook, instagram, or twitter feels more like I won't be able to recreate with my friends/family and less like I won't be able to communicate with them.

I think the MMORPG example is off for a few reasons:

1. The primary product of an MMORPG isn’t access to a social network.

2. The social networks in these games is far smaller than social media platforms (we don’t usually worry about anti-trust for tiny markets)

3. As you mentioned, social media platforms are communications/infrastructure, not recreation.

> 1. The primary product of an MMORPG isn’t access to a social network.

Exactly. You can switch to another MMORPG and find another level 29 healer to restore your HP, but if your mother only uses Facebook, it's much harder to switch to a different social network and find another woman who gave birth to you.

This is a definition that is broad to the point of uselessness.

Regardless of what program I use to communicate with my friends, that program is now a monopoly. Even if I coded up the website myself and my friends and I are the only ones to use it, that website now "owns access" to all my friends. It is absurd to label this website a monopoly.

This is also not actionable. Once my friends and I all decide on a communication platform, that platform is a monopoly and must be broken up (?), by removing some of my friends from the platform (?). In that way, it would literally be impossible for me to have a platform where I can communicate with all my friends, because such a platform would be a monopoly and must be broken up by your definition.

1. No one cares about monopolies on tiny markets.

2. It's perfectly actionable: open protocols separate the platform and the network.