No it is not shirk. When asking or answering for points, then there will be no reward even if it benefits others, while asking and answering for the sake of Allah with the right intention than that Insha'Allah is rewarded. It's all about sincerity here, but if a user starts worshiping the points by making it his only goal to get as much of it and by letting go of worship like salat and other things just because he is obsessed with this site, than that is delving into shirk because he is taking the points over Allah. Other than that, if one is not obsessed to that degree (which I doubt Insha'Allah anyone is) than it is a matter of lack of sincerity and the result is only no reward, it renders it useless.
As for trying to stop a possibility that might happen, I don't think making upvotes invisible to others is the answer, because upvote and downvote show the quality of it. Rather I think either of two things should be done:
- No reputation
- Make reputation of a user invisible to others except for the user himself or herself
Both of these will have bad results, for if number one is to be implemented, we won't be able to tell who is worthy for certain privileges and who isn't. That means anyone can come sign up and be able to do what only a trusted member can d, delete, close, and other things which might lead to major kayos.
If the second option is implemented, than it might discourage others from even answering. So the best thing to do is leave it up to the User and God, because if it is Allah's will that someone come here and be lead astray, no matter how hard we try, we won't be able to stop it.