Karma is another forms of Merit Reddit use to qualify or reward a user who make a good composition of write-up or communication, so this is also related to Facebook likes, which Facebook user's use like to appreciate someone images or postures in picture, and also a text of someone.
From my interpretation, and understanding of Reddit's up voting system is its to up vote things that you believe deserve more visibility. Remember, that Reddit isn't by default sorted in chronological order, and while it can be filtered by new, most sub reddits are defaulted to "hot". Thus, whatever you upvote has more chance of being seen by others.
Merit isn't comparable since merit is less prone to abuse, doesn't effect visibility (you could argue that merit posts stand out slightly more, but it still doesn't effect the order of things), and is given to reward those for making substantial posts.
Reddit's system is therefore not even comparable. Remember, that anyone can upvote including those that just signed up. So, theoretically you could inflate upvotes or downvotes simply by creating an account farm. Here on Bitcointalk the merits are limited, and by design less merit per merit sent. Of course, we have merit sources which keep injecting a certain amount of merit each month based on theymos' calculations, but
normal users are receiving half of a merit for every one merit they receive.
