Social Engineering is a broad term, but I know that I've worked for a few companies doing investigative work and social engineering is hands down, the most valueable tool.
I worked for a company that tracked down people owing large amounts of money, professional debtors and frauds, and we pretty much had to rely on the human element to ever get anywhere, because most info reported in the standard governemnt files are old. Credit bureaus are old. Property records, etc. When people want to hide out they rent rooms, never open up any utilities or even mobile numbers in their own name (their credit is totally shot anyhow).. they work under the table or take on jobs that don't do legit checks and fudge a digit on their social if they don't already have fake id's and crap. There is a thing called "spoofer" that you can get where it allows you to enter the number you want to appear on their caller id, so you can call and claim your somebody else looking for them. If you think you've got a good number for them (by doing a cell break so you can listen to their voicemails) you can then try calling them from an old employers number and claim you need to send them a W2 for a new address, but most of them will have mail sent to family members. I've opened up fake facebook and myspace accounts appealing as a young, cute, party girl just to get a friend acceptance, then stalked their page and their friends and familys pages to get an idea of what they might be up to. BUt the human element is always going to be the best. I don't claim this type of work or behavior is very noble, and I don't do it any longer, but it has defiantely opened up my eyes to just how much info you can collect on someone if your willing to put in the time and money.
On the flip side I work from home for an airline and found out about a year ago that some of our employee files were compromised, and that some group (whos objective was not disclosed to me) has been caught going to our physical addresses and digging through our garbage in hopes of finding some kind of info. Its a crazy world we live in.
Man suffers only because he takes seriously what the gods made for fun - Watts