The music industry is one of the harshest, most demanding, unsupportive, emotionally exhaustive and flat out difficult fields you could enter. But the payoff is... well you know.
You have to have thick skin to survive in this industry. People say this pretty much for any creative field because you put your heart on the line and it gets mangled, squashed and vampire sucked dry on a daily basis. You have to accept that there will be haters who are so insecure with themselves that they use you as their punching bag to take out their directionless frustrations and uncontrollable jealousy.
You know you've made it (at least on a small scale) when the haters come out. No matter how much of a saint you are in person or how talented you are on stage or how undeniable your recordings are, your illuminated existence will be off putting to some and you just have to accept this.
There are a few ways to approach this. You could, every time you see a negative post about you on Facebook comment back, personally attacking the hater's own music tastes and wardrobe. Hell why stop there? Go into the history of his photos and find some ugly chick he had his arm around once and go nuts. OR you could ignore it and let karma take care of him. Or, if something really bothers you, you could approach it with a polite, private message.
I once received a 2 star review on iTunes where the reviewer called me a "tool." Outright! He did include a compliment saying that an old song of mine that he saw me perform live was "one of the greatest live performances he'd ever experienced," but then added that my studio albums were such a disappointment compared to my live show. He went on about how my new album was a slight improvement, but not by much.
Thanks to iTunes Ping it listed the reviewers full name. I found him on Facebook and started a very polite, very friendly conversation. I asked him if it was really necessary to call me a "tool" on iTunes and told him I'd give him a full refund for my previous albums that he had purchased, but didn't care for. I also asked him if he had purchased my new album and listened to all of the songs or just the brief iTunes previews because my new album did include aspects from my live performance that I knew he would appreciate. He wrote back, tail between his legs, and apologized for calling me a tool on iTunes and conceded that it wasn't so tactful. He also admitted he hadn't actually listened to my new album in full and did only preview it briefly. We went back and forth a few times and the conversation ended by us discussing music and trading new bands we like. He bought my new album, revised his review and the next time I played his town, he came up to me after the show, handed me a beer and introduced himself.
It's so easy to take every criticism personally and fight fire with fire, but if you can keep your rage under control and approach petty insults like a mature, self-assured artist, you will not only be much more respected (who wants to witness a cat fight on Facebook by an artist they thought they looked up to), but will gain many more fans in the long run.