Before they went public, who was foolish enough to invest in a company that has never turned a profit?

You'd be surprised. The basic strategy of losing money hand over fist for years to grow yourself to as large a user base as possible, before finally aggressively monetizing that user base, is well established in silicon valley. Investors would not even raise an eyebrow at the loss numbers posted by Reddit because of how exceedingly common that is.

And it has worked several times, making some people ridiculously wealthy. Good examples are Amazon, Facebook, and Uber. So usually companies on this level have raised hundreds of millions to sometimes billions of dollars in investment capital, allowing them to operate at these levels of losses for years at a time.