My first job out of school required a security clearance. A few years later, the clearance required an "upgrade" (To quote Forrest Gump, "That's all I have to say about that"). I have never had a large social network, and I don''t think it hurt my chances of getting the clearances.
I have no direct information on the subject, but I think there are a few types of people the investigators identify as potential risks. These, obviously, include people who might have an idealogical reason to divulge secrets to folks the gummnt doesn't like. If you were a member of the communist student club, or spent a lot of time in countries on the US "do-not-trust" list, it might hurt your chances.
Another risk category are people who can be extorted. That could be someone having an illicit affair, a closet homosexual, or someone with "something to hide" that a foreign agent could exploit.
Another risk category would be someone who finds himself in a situation where he is so desperate for cash that he would sell secrets to cover his debt. If you look like you might have a gambling problem, or have lots of debt (I wonder if student loan debt counts?
) you could be a risk.
Of course, this is all speculation, but most of it is based on the questions that investigators asked my neighbors.
In the interest of full disclosure, I havent had a job that required a clearance in quite a while.
ETA: I agree 100% about what lightning bug said regarding honesty and accuracy.