They may look everywhere. However FB represents a single large concentration of microbloggers. TLAs can get a direct connection into Facebook's computers to filter all new comments.
To do the same with FR, DU and the like they would need to scrape millions of pages on thousands of blogs and deal with a whole bunch of related difficulties. Additionally, 3rd party blogs like FR give you no easy way to associate the login name with a real life identity. Facebook most likely includes the IP address with each comment and provides the account data if the TLA wants to look deeper.
The same effect occurs in crime. Majority of crimes are committed by low IQ people within a mile from where they live. (Trayvon Martin is a good example.) Detectives know that and use it to solve those crimes. A few crimes do not fall into that pattern; for example, a hired killer can assassinate a businessman. Such crimes are hard to solve even if you have a list (a long one!) of all the enemies of that businessman. The hitman is experienced; he did everything right; he is not known to the police; he cannot be associated with the victim; and so on. The police sends resources (detectives) to where they can be most useful. In case of FB vs. FR that would be Facebook.
Okaaay. (Waving at Greysard:>))