Indulge my hypothetical:
1. A newspaper article is posted at 1PM.
2. A suspicious comment that contains nonpublic, incriminating information is posted on a message board at 1:15PM. That comment links to the USA Today article. The message board is outside of FBI jurisdiction.
3. The Feds already know the identity of the commenter through an illegal and/or unconstitutional means of data gathering. (Think of all the secret taps built into AT&T's networks, their Stingray devices, all of the unpublished vulnerabilities exploited every day by TLAs, etc.)
4. If the Feds can get the identities of everyone who read the article during the window immediately before and after that comment's submission, they may be able to use other information to narrow that list down to one or two suspects and use that to sway a judge to issue a search warrant. Because they already know who they're looking for, I wouldn't expect this "narrowing down" process to be particularly even-handed.