For some members of the original Chinese "50 Cent Party" it was a full time job, receiving up to 50 cents (two yuan) each for up to a hundred pro-government messages posted a day, using several dozen different accounts. Eight years ago, when Chinese propaganda officials ... organized the pro-government posters already out there. The propaganda bureaucracy (which is huge in China) did so and got so many volunteers that they soon developed a test to select the most capable posters, and also set up training classes to improve the skills of volunteers. Cash bonuses were offered for the most effective work. At one point, the government had nearly 100,000 volunteers and paid posters operating. This quickly evolved into the 50 Cent Army, and now the 50 Ruble Army in Russia.
|Cleaning spitoons, DC 1914|
In the US this is mostly from private enterprise but includes a stable of tame political bloggers who help out. Think JOURNOLIST and look for a spitoon to get the taste out of your mouth.
Wikipedia has a more scholarly review of the 50 cent party (army).
China has lots of businesses in the same swamp as their government: "One firm recently admitted to employing more than 800 people who created nearly 20,000 fake user IDs. Half of the posters create at least one post or comment at the rate of one every 2.5 minutes, and many do this full time. And there are many such PR firms engaged in the practice."
UPDATED: See comment #2 from Osaka48. Chinese trolls (a foreign power) meddling with the comments at the Wall Street Journal.
Updated February 2013: More information from Strategy Page. "Internet Apes".