Say, just for the sake of argument, that you attacked Bill O'Reilly's web site and took account data that included credit-card numbers, Social Security numbers and bank accounts.
If three or four years passed and you hadn't heard from investigators hired by Fox, odds are, you never would.
In November of 2010, the FBI raided the dorm room of University of Akron student Mitchell Frost and used a disk it found hidden above a ceiling tile to get him a sentence of two years in jail and fines totaling $50,000 for attacks on O'Reilly in 2006 and 2007 and hacks against the University of Akron.
Fourth (and weakness No. 1): the FBI has limited attention, but lots of friends:
The FBI wouldn't have nailed Frost for the O'Reilly attack if he hadn't also been under investigation for other attacks. He got on the radar of IT people at the University of Akron by launching malware and penetration attacks through their network to other sites and, at one point, taking the whole network offline for more than 8 hours after attacking a game server in the U. Akron library.
A network administrator started collecting activity logs on Frost, reported the attacks to University Police, who reported them to the Secret Service, which brought in the FBI.
So, even hackers who manage to keep the feds from noticing them – or eluding capture for long enough that it's not worth the time of individual agents to keep pursuing that one case -- have a good chance of being dimed out for other things
None of those friends – whether commercial security firms like Kroll or humble network admins from your alma mater – has the kind of power the FBI does. They all have one power it doesn't – they're a lot closer to you, are much more likely to realize what you're up to, and are able to bring the feds down on you with a phone call.
And if it's the IT people you piss off, there will be some hard evidence of your activities from server, network and firewall activity logs – information the agents themselves may either never request or may take far too long
Fifth (and weakness No. 2): The FBI's short attention span is focused by outrage
There are a lot of crimes of all kinds going on all the time, far more than even the FBI has time or resources to investigate.
So it has to divide its time according to priorities that sometimes favor hackers by pushing them far down the list of potential public enemies, and sometimes punishes them unfairly by raising hacking or hactivism to an unreasonably high profile considering the amount of damage it does.