Some solid points there, Smillew! And I don't really disagree. If you're just starting out, you probably shouldn't spread yourself too thin.

That said, I don't think it's the number of followers that should determine when you start exploring other platforms.
There are plenty of awesome writers here that have been writing for years and that for whatever unfathomable reasons can't get any traction (even though their stuff is solid). If they wait until they reach 10K followers (or whatever number), they may never give another platform a shot.
So in my pompous opinion, it's when you have enough experience and content you can easily repurpose that exploring other places makes sense.
Also, don't underestimate first-mover advantages. The sooner you get to a new platform, the easier it should be to build an audience there (even though it will still largely be a crapshoot, as all algorithm-driven popularity contests are.)