Considering social patterns, people are much more inclined to up-vote or like things that down-voting or disliking. Which is the opposite of the normal behaviour that we expose on a day to day basis where people complain more than acknowledge good things.
In social media, or sites that use social behaviours, the fact that people tend to feel bad about criticising is used to their advantage, plus the main interest of those sites/groups is to get exposure; it's much more probable to get exposure because you liked something and showed it to some body, not to mention the obvious that they want good publicity, not bad.
Also, remember that we, as humans, are biased by what other people do, if we can see that other people are voting things up, there a re more chances that we will do the same. That also applies to down-voting. So if you combine the intention of exposure and the copying/biasing behaviour, you have an interface that has to be designed with nice evident colours for voting up and not so bright colours for voting down.
All of this, of course, applies as a general rule, specialized sites with specific rules, prices or recognition systems work a bit different.
