No, I don't think we all know this. Most of the people I know did not get jobs through family connections. I suspect a trend analysis would show that the number of high-income workers who achieved their position through nepotism or some other form of non-achievement-based social connection is declining, not rising.
We're social animals, and "connections" will always be valuable. I don't see any evidence that D.E. Shaw treated Wyden's son differently because they wanted something out of his father.
Seriously though, after working at a prestigious institution and seeing how often lunch and dinner conversations between peers involved talk of how "my daughter can work in your group!" and "your son can work in my group!", my intuition tells me this is just the way it is - nepotism is pretty much everywhere. Sure, more anecdotes, but that's all I got until someone does some solid research on it.
For now here're two more articles, that may have some more information: http://www.washingtonpost.com/blogs/monkey-cage/wp/2014/01/3...
http://www.theatlantic.com/magazine/archive/2003/07/in-prais...
There's no requirement that data must be exhaustive. And in most social science research, it never is (actually that is frequently the case in hard sciences, too).
We all know how many people get jobs because of connections... the privileged among us just take it for granted -- indeed because it so unremarkable we often just forget how we got there or don't think much of it (I'll confess that I initially got a well-paying job (that I was entirely unqualified for) purely because of family connections). Just think how many of these backroom "hey instead of paying me $2000 for my campaign, hire my daughter who's having trouble finding employment" dealings there are that go unreported and are never found out. I'd say it absolutely is commonplace.