"A random person on the internet is posting creepy comments" is likely not probable cause, especially as you'd have to convince someone national like the FBI to care without a specific location.

"Someone I know personally is posting creepy comments, and I've always gotten a bad vibe about their interactions with kids" is probably enough to get local cops interested enough to knock on a door.

I hope not. Unless there is evidence of a crime that has occurred or evidence that a crime is planned, they have no business (constitutionally) bothering a citizen based on that kind of tip. This is how lives are ruined when there are no victims.