It’s not quite that straightforward. There’s some non linear network effects – as more communities emerge, each instance is likely to be pulling content from more and more communities. Thus the storage costs along will baloon, and the instance to instance communication will baloon, but not necessarily scaling with user count. It’s not likely to be scalable in the long term, if my math is correct.

It’d be like each subreddit having to make a copy of the entire reddit database, over the network, to participate in reddit. Works on a small scale. Fails badly if the community is too large.