Whether to mention contributions on Stack exchange in Statement of purpose (SOP) for MS/Phd application in US?
My rough answer would be that you need to convey to your reader:


What SE is.  Many people think of it as a place to cut and paste answers without thinking too hard, but the community side isn't as well known
How to indicate your interest/passion.  The reputation number is fairly meaningless to an outsider.  Possibly a statement like 'contributed daily for 5 years' has more weight.


Personally I'd view it as a plus but not a massive benefit.  It would be more remarkable at undergrad entry to show engagement in the subject, but at masters/PhD level I'd assume you're already engaged.  It does show interest in teaching which could be positive (eg if you're likely to be a TA at some point).

I'd weigh carefully what you convey by this statement against the space needed to convey it: it may be there's a more impactful way to use the space.

[Disclaimer: I come from a UK perspective.  Other countries may be different]