It's simple, eyeballs, ads, and data. They can directly measure the value created through ads posted on those properties - and that scales up with the amount of potential ads. Then you have future value of the data being generated (personal habits, info on Facebook, watching patterns by demographic etc on YouTube).
For Reddit, it's one of the biggest communities online (maybe the world?) - there are many ways to leverage and monetize that even if they haven't figured out all the optimal ways to do that yet, the perceived value is enough to convince investors in its future.
For Reddit, it's one of the biggest communities online (maybe the world?) - there are many ways to leverage and monetize that even if they haven't figured out all the optimal ways to do that yet, the perceived value is enough to convince investors in its future.