oh, I mean ASIO.

Folly does not use boost::asio, but rather libevent:

  libevent is an excellent cross-platform eventing library.
  Folly's async provides C++ object wrappers for fd callbacks
  and event_base, as well as providing implementations for
  many common types of fd uses.

https://github.com/facebook/folly/blob/master/folly/io/async...

grepping the codebase for `asio` brings up nothing.

ASIO header-only implementation is also available without Boost, fyi, as a standalone library; the same one that's in boost, but used without boost.