For me, it's largely replaced Twitter already.

I mainly used Twitter to see what was going on in tech & open-source and discuss development topics, and comfortably 50% of the tech people I followed on Twitter have now set up on Mastodon, either additionally (cross-posting to both) or in quite a few cases as a full replacement. I post the same thing to both and I get far more responses & engagement on Mastodon now. For me personally it _has_ replaced Twitter.

Depends which circles you move in and what you use Twitter for (it's certainly not useful in the politics & journalism scene yet, for starters) but there's already significant movement here in some areas.