Computer security is obscenely asymmetric - an attacker only has to find one flaw, once, somewhere. A defender needs to constantly monitor, test, review isolate and basically never make any mistakes. It is easy to look at almost any intrusion and attribute it to poor defenses. If HBGary didn't have a SQL injection, they'd have had a XSS vuln. Or a employee would get spearphished. Or an attacker at a local coffee shop…
That is something the IRA used to say, they only needed to get lucky once, whereas the police needed to get lucky all the time. Of course humiliating someone on the Internet is a world away from blowing up a shopping centre. If the consequences were more serious than embarrassment, then a lot more resource would go into guarding against it. Schneier talks about attack trees (http://www.schneier.com/paper-attacktrees-ddj-ft.html) - always look for the cheapest vulnerability.
Incidentally there is one online group who could eat Anonymous for breakfast - Mumsnet. If Anonymous ever took them on, they'd be grounded before you knew it.