I read the web site, and I still don't understand what this web site is all about. Is it really just yet another messaging platform designed to get around SMS messaging charges? Am I missing something obvious?
1. There are tons and tons of ways to send messages to people last I checked. Why is this one worth "$16B"?
They pay for the userbase.
2. Who still pays for SMS messages? I've had unlimited texting plans for the better part of a decade, and they're cheaper than most people's cable TV bills. Are text messages significantly expensive outside of the US?
Yes, outside the US prices vary a lot. I pay, what you'd percieve as 20-50cents per message. I know other countries do have free SMS. Some plans here have free SMS, but they're the extremely expesive ones.