I think all Social Networks are biased, and most of the time, it’s a positive bias.
I am not suggesting that there is zero negative comments online; that’s obviously not true. What I am arguing is that people tend to create and/or share things on Social Networks that can somehow help them to build their “online social status” in a positive way.
Take Yelp.com for example, the characteristic of Yelp is biased heavily toward a remarkably great dining experience. Sure, there are complaints about crappy restaurants, but the majority of the reviews are either good to great (4 to 5 stars). This is especially true for popular restaurants.
I make the assumption based on this simple idea: a 3 star restaurant is not really worth mentioning, and you would appear kind of stupid (to your peers) if you keep eating at places which you rate 1 or 2 stars.
So here is the question you should ask yourself: if you want your customers to talk about you on their Social Networks, what can they say about you that would make them look good in front of their peers?