The self hating American or Westerner - person who talks obsessively about the decline of the West, but claiming to be from there.
The US politico - Taking an extreme position on US right or left wing politics and using it to distract from discussions about China or good news about the US or the West in general.
The "Poor" American - A person claiming to be an impoverished American that is part of a wave of poverty sweeping the nation (though they are on the Internet posting comments on news stories).
The US radical - Taking an anti-government position and calling for the ouster of both political parties, with a very cynical outlook that nothing can ever improve.
The racial stereotype - They will pose as different races, claiming bias or racism on articles that have nothing to do with race. They will do this to make the conversation bitter and unpleasant on something that was otherwise a good story. They will often use outdated racial terms and grievances that no longer apply.
The racist - They will pose as racists, saying incredibly racist things to destroy a good discussion that is pro-West or anti-dictatorship. They use stereotypes and racial terms that are outdated and English grammar that is strange, and so they are somewhat obvious.
The angry Muslim - They will pose as angry Muslims, and add to a chorus of anti-West sentiment. Though their use of language and Islamic terms is odd and seems scripted.
The disgruntled Native American - They will pose as Native Americans but not convincingly, as they do not seem to understand their culture or the current reality of their integration in US society. They will of course, have nothing but angry tirades to say but it will not be consistent with the views of real Natives that Americans know in person.
The flame starter - They will start pointless arguments in a range of discussion forums (Apple vs Microsoft vs Google), gays vs straights, religious vs non-religious. All given away by unusual English usage and outdated/absurd discussion points.
The divider - They will try to divide nations of the West or potential allies against China. They will post as Americans mocking Indians, Indians mocking Americans, Europeans mocking Americans, and so on and so forth. Using racial and cultural stereotypes to cause further argument and disagreement.
The dissident - They will pose as Japanese or South Koreans that are pro-China, and want the US military to leave Asia.
The doubter - They will cast doubt on US military capabilities, US economy, US stability. The same with Europe on articles relating to Europe.
The exaggerator - They will exaggerate the military and technology capabilities of Iran, North Korea, and China. For example, claiming that North Korea was capable of sinking a US carrier and trying to cause worry and doubt in general.
The weak allies - They will pose as Americans, seeing no need to provide security for South Korea, or being unwilling to help. And suggesting we abandon our presence in Asia.
The killjoy - Articles of good news get dismissed as lies or exaggerations. Or various means are used to distract from the good news. On many youtube videos (especially those with 1 million+ or trending) they will start completely random racial or nationalistic arguments that have little or nothing to do with the content of the video. Or they will argue different genres of music, in a very exaggerated and confrontational way that includes cues from poor English grammar usage.
There are many other types of posts but these are some that are somewhat common in US social media right now.