Why do they need All The Users In The World? Why can't more services be content with having a significant and active user base that is not the entire population of Earth.
I think because the network effect is a fundamental foundation of social networks. Think of all the SN's that have come and gone. It appears you can't really remain static with a non/slow growing user base. Eventually, something else comes along that has a broader range of people making the content better; you stop opening the older, more static app.
There are lots of dedicated, niche forums all over the Internet that have remained relatively stable for a decade or more. Those are social networks, and they work fine while accepting the users they have.