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.)