A person (usually a guy) who either a) has no friends, or b) shields his friends from others all the time. If you meet one of these people, you won't be making any "friends of friends" through them anytime soon, no matter how hard you try or how many years pass. If the SDE is dating someone steadily, you won't even know her name until after they break up.
A SDE will especially shield contacts of the opposite sex, even if they have no chance with them, for fear their friends will "steal their girl/guy". If they're in a steady relationship, they still shield because a) they're still afraid someone (including you) will try to steal her/him away, or b) the SO is nothing to be proud of (ie a fugly ditch pig or pork chop). When a SDE brings out a contact of the opposite sex, it's almost always an ex, or someone he/she tried and failed to get with. Meanwhile, the SDE will have no problem asking you to introduce more girls/guys to him/her regularly. If you do it's a one-way street.
If you are stuck in a social circle full of SDEs, you must find some non-SDEs on your own, as your friends sure as hell won't be helping in this department.
Engineers and Computer Programmers come to mind.
After knowing Vic for 4 years, Sam only knew of some girls he tried to get with, but not by name. At that point Sam realized he'd wasted 4 years of his life hanging with a social dead end.