The homeland security department of US continuously makes fake profiles in Facebook and Twitter as a method to track the people who are using sensitive words.
If they find any user to use a phrase or word from their department's list, the US spies will read the posts and then investigate the account of the user. They also many attempt to find out the person handling the account.
Some of the words that have attracted the attention of the spies are trojan, collapse, deaths, recovery, virus, illegal immigrant, strain, drill and outbreak. This was a report made by a private group and it is not clear on how this information is used by them.
The group claims the US government to use scans of the social networks earlier in order to analyze the specific events like the BP oil spill in 2010.