Regarding your comment that "It will be interesting to see if a profit-based variation of Zidisha emerges...": it seems to me that you have a positioning dilemma. The minute you invite someone to lend for interest, you are profit-based. The fact that you cap interest rates, perhaps requiring that lenders charge far less than risk would otherwise indicate, doesn't change this.
If your lenders aren't motivated at all by profit, don't care to allocate capital in a manner closely aligned to risk, and you don't see yourself as being profit-based, why not simply allow me to donate money to specific individuals? Being able to target small donations to a specific person for a specific has a lot of appeal. In fact, there are certain charities I'm familiar with that I know frequently get requests from donors who want to be able to trace their donations. Unfortunately, tracking individual donations to individual recipients is something that a lot of major charities cannot do efficiently at their scale. It would simply cost too much.
Obviously, the lending aspect of your platform allows me to redeploy my capital, something I wouldn't be able to do if I was simply making a donation, but as with any other P2P lending platform, my interest earned is taxable income so contributing through Zidisha is far less tax-efficient than donating to a 501(c)3.
Here's an idea which may or may not be feasible legally/logistically: I donate $50 to Zidisha, which Zidisha loans to a borrower of my choosing. When that person pays back the loan to Zidisha, Zidisha allows me to direct the lending of $50 plus the interest Zidisha earned to another borrower. And so on.