It's only slightly misleading to say Kiva lets you lend to small businesses, but downright false for RWW to say it lets you invest in them. If you can't get any return on the loan, it's simply not an investment.
I think that is the purpose of micro-finance. http://en.wikipedia.org/wiki/Microfinance
Lending is the most common form of investment, and it's comparatively safe. Equity investment is what's risky and dangerous (unless you get into junk bonds, or lending to bankrupt companies).
"Lending to the working poor through Kiva involves risk of principal loss. Kiva does not guarantee repayment nor do we offer a financial return on your loan. "
http://www.techcrunch.com/2008/11/26/sec-outlines-its-reason...
I'm not sure where they sit now, but when you browsed Prosper you could find people requesting loans for business (not just personal items like cars).