You could do an informal survey asking people what portion of their "nest egg" they've invested in Prosper, or if you wanted to get at it differently, what portion of their income they invested. Of course, that's all self-reported so not the most reliable info, but it's what you have to work with.
(I was going to suggest going by how much people have invested, but I think that's more often going to be an indicator of caution than $$ available. There could be a number of people who have great pots of money but looked at how new and potentially risky Prosper seemed, and only tossed in a small amount at first, as a test to see if it would be a viable investment. Then of course they learned more about Prosper and declined to put in any more.)
This is a good idea, but I bet a lot of Prosper lenders (fewer at .org than elsewhere, but still a lot) would not even know the answers to your questions... I got the definite impression that for many smaller lenders, prosper was their first investment vehicle of any kind... A lot of them probably don't have budgets themselves, let alone a knowledge of portfolio allotment, et. al...
Your second paragraph is also true. While the "how much are we worth?" thread suggests theat .org members have lent several millions of dollars, perhaps as much as 5-6% of the total amout lent at Prosper, maybe even more, I know I didn't put in a quarter of what I was contemplating putting in, simply because it didn't feel safe. To me, putting money in a choppy, up-and-down stock market during a bad economy is safer than putting it in Prosper.
When all is said and done, I have about 3%-4% of my investment assets tied up in Prosper, which seems like too much now that I look at that firgure. As for portion of income, that is very misleading. Beginning today (YAY!) I am taking a semi-sabbatical and switching to morning work only. My plan is to never return to fulltime work again, except for myself... So as a portion of my annual salary, the Prosper stake will look inflated.