Reddit doesn't really have network effects the same as Whatsapp and Facebook.
When I say "network effects" I mean two things:
1, A self-fulfulling draw
2, An increase in the utility the more people exist
So, having everyone on whatsapp means you can have group chats.
Having everyone on facebook means you can plan events.
Reddit kinda doesn't do that, in fact it optimises for a loose coupling of many independent and semi-isolated communities.
It doesn't really take advantage of any network effects; and like Lois Rossman mentions: there is next to no benefit being on reddit than using private forums except having a single login. Which is essentially a solved problem these days because SSO is easy and ubiquitous (thanks google/github!).