No one, including social media sites, is obligated to provide anyone a forum, anonymous or otherwise.

Further, no one is immune from the consequences of their actions. If you express pro-slavery opinions, the local groceries are not obligated to continue stocking your barbeque sauce.

http://news.google.com/newspapers?nid=1876&dat=20040317&id=okQfAAAAIBAJ&sjid=P9AEAAAAIBAJ&pg=6474,649936

The local villagers can't burn your castle, but they don't have to continue doing business with you.