SoFi (NASDAQ: SOFI) recently reported its fourth-quarter earnings, and the growth in its lending business has been impressive. In the fourth quarter alone, the company produced loan originations of $10.5 billion -- its highest quarterly total ever. While this sounds like a massive amount of loan volume (and it is), it could just be the beginning. In fact, SoFi's loan business remains a multi-trillion-dollar opportunity, and that's just in the three types of loans the fintech currently offers. Where to inv ...