Assume that a student has a continuous involvement in any community of Stack Exchange.(consider 1,000+ reputation).
Does It has a positive impact to mention this involvement in order to show you are passionate about that field? I am asking this because the Statement of purpose has a great impact on your MS/Phd application.
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]