I think complaining is valid if you know HOW to complain. Venting about a faulty product or bad service on your own page on FB is a waste of time because it isn’t going to achieve anything. Instead you should get onto the retailer’s website OR FB page and complain to Customer Service. They usually respond very quickly because your complaint has been made public. I have done this when I was dissatisfied with a service and it was very effective.
I also don’t think that just because you post negative comments on social media that you should be perceived as a ‘negative’ person. It would be unrealistic if we were all ‘happy clappy’ people and agreed with everyone else all the time. There needs to be balance in arguments, so you need to see the positive as well as the negative.