Twilio makes its money on SMS and voice services. Those services are cheap and very easy to develop with using Twilio.
Note though that most cellular carriers are moving towards data plans instead of voice packages (Verizon especially) because most people use their phones/mobile devices for data consumption, and less as phones. So while I think Twilio provides fabulous value now, I feel like they're at the tail end of the telephone-based telecommunications revolution. As people move more away from POTS calls (and more towards Skype, Google Chat, iMessages, etc), the need for phone calls and SMS messages will drop quite a bit, as they'll all be handled over other services.
TL;DR Twilio has a shelf life. It'd be like developing services around a shrinking but still necessary industry (like the postal service).
Disclaimer: I love Twilio. I went to last year's TwilioCon and have met management.