The point of the threshold isn’t to tell how many Minnesotan kids can be on a service. It’s to limit the burden of filtering traffic for Minnesota to companies that should be large enough to manage it. A small social network may not have the engineering resources to devote to doing this, or it might be cost prohibitive. This threshold mitigates those complicated arguments to avoid the filtering.

Also, a new social network wouldn’t be as likely to be seen as a “threat” until it got to a certain scale.

(I’m not trying to defend the bill, just what I think the the rationale is for the thresholds used)