I'm curious to know whether someone able to see the person user id or name who downvoted their questions or answers in salesforce.stackexchange.com?
Neither Moderators nor normal user can see that information. Which .. creates anonymity and true freedom in giving your votes. Which kinda is a good thing I'd say. Even Though it's often appropriate to comment when you're casting a down vote.
You can however see how individual users are voting overall and how distributed they are:
on a user's profile:
Actually, I believe moderators can see voting history. If you look at the help section, there's something called serial voting which can be reversed. That applies to both upvoting and downvoting the posts of a member that someone might be following on SF.SE either as an angry stalker or as a fan.
More to the point, when I first joined SF.SE and made my very first post here, someone was admonished in the comments for having downvoting my post. That couldn't have happened if no one had access to or could view other's votes. Its possible that may have changed since I joined a year and a half or so ago and if so, I'll stand corrected. However, its clear that at one point in the past, it was possible to view those things, either that or someone was very astute in noticing a -1 point change in another member's reputation; something I find a bit hard to believe.