During a classic South Park episode, Cartman dresses smartly to impress Kyle. When Kyle retorts "that's not being nice, that's just putting on a nice sweater" Cartman replies that he doesn't know the difference. "I know you don't", Kyle says.
We can claim whatever we like about ourselves. We can claim to be fantastic bloggers, or inventive, or impartial. Our peers judge the validity of our assertions by assessing our actions. "Ruben claimed he's a fantastic blogger, but Rubenerd.com is absolutely terrible!"
Make false claims about yourself repeatedly, and you lose credibility.
Niceness is one such judgeable (judgeable?) trait. It doesn't matter if your social network profile has a unicode peace sign, or if you append every sentence with a smilie. If enough people feel slighted or burned by your remarks, you'll develop a reputation contrary to what you claim about yourself.
"Just got off the phone with some fanboy. What a tool! :-)"
Fortunately, there's good news. If you're nice, people in your circle won't need you to tell them.