Obviously, firewall is not important to avoid monopoly and those American companies which lose the Chinese market has little connection with the firewall. You can see when Google decided to leave China in 2010, the Google's market share is about 30%, less than half of the market share of Baidu, a local search engine. Ebay and Amazon entered into China early and there is no local companies with enough power to compete with, but now Taobao and Jindong dominate the most online business other than those western pioneers. There are also some other examples, like Yahoo vs Sina, MSN vs QQ and etc. All these are not the result of the firewall but the result of the free competition because there is no firewall when the competitions between these companies occurred in China.
But I will admit that the firewall benefits some local social media companies like Youku, Renren, Weibo and etc, their competitors like Youtube, Facebook and Twitter have been banned in mainland China due to some political reasons.