More than you know. The absolute best way into a target network is through sending phishing emails (which is just social engineering), with links to documents or such that exploit the target system. Firewalls prevent external attacks, they are notoriously bad at stopping inside systems from calling out (by design). So if you can get a user to call out- or go to a website outside like this Wal-Mart guy did, then you can generally own that system.
Its always best to send emails that the person would open, so knowing your enemy is important. if you take your time and do your research its not difficult to learn the entire chain of command. Then there are email proxies out there that let you send emails that look as if your coming from any address you want.
so you find out who the "targets" boss is and send them an email saying urgent please read (and put an attachment that exploits adobe or word etc etc and you own the system. This works best if you find a boss that is over many people, all you need is one to click and your in.. It ALWays works, I've never seen it fail, ever.
Not that I know anything about this kind of thing or have done this ever
