I think your argument works better as s comparison between mediums, such as telephone vs. letters/meetings and telephone vs. internet. Despite what they would like, FB is not the medium.

Network effects aren't just between mediums as you are defining them: one might call “the internet” a medium, but it's clear that someone using the internet but not Facebook is as locked out and disadvantaged if social communication is occurring on FB as someone who isn't using the internet at all.

It isn’t clear to me at all, given the wealth of options. Someone using an encrypted messenger for example, strikes me as being quite connected.

> Someone using an encrypted messenger for example, strikes me as being quite connected.

To the network of people using a compatible encrypted messenger, not to people using "the internet" in some generic sense. Network effects definitely apply to each of the options at the level that they have their own network (e.g., "email" is a network, to the extent that it is interoperable; "Facebook" and any particular encrypted messaging system are each their own networks. The utility of each is determined largely by how many and which other people are using them and what those other people are using them for.)