http://www.slideshare.net/twilio/reinventing-the-dialplan-sl...
Plus they have said publicly in other places that they built upon Asterisk.
As for Freeswitch. It is a great platform. Both Freeswitch and Asterisk may be made to scale, but it is not trivial and takes a fair amount of time and resources to do so and then to maintain over time. The issue for both of them is that the applications and media services tend to run in the same process. Digium's answer to this is SCF which is now under active development.
The approach we take (and others like Oracle, JBoss, Avaya, etc) is to deploy apps in SIP Servlet containers (Java) and your media in dedicated media servers (C/C++ for example). Employing a clear demarcation between application logic and media processing using MRCP (http://bit.ly/32Bnpu) between them.
Now, of course Freeswitch does have 'Mod unimrcp' (http://bit.ly/hmqvdq) which may be used to talk to our media servers (http://bit.ly/f9lUH3) and turn Freeswitch into an application platform. But when most people think of Freeswitch, they think of the equivalent of Asterisk and deploy that way.