The challenge stems from orthogonal demands: 1) scaling alienates early-adopters and 2) scaling in response to overwhelming demand exacerbates the onboarding headaches and undermines user experience.
That's where Mastodon is *right now*
Where Mastodon goes largely depends on whether the UI/UX issues can be addressed by a 3rd-party client, like the much-anticipated @ivory (h/t @paul) *and* how well large Mastodon instances handle surges in traffic.