https://js.braintreegateway.com/v1/braintree-data.js
produces a function called createThirdPartyBlock, which (upon calling BraintreeData.setup) injects
https://www.paypalobjects.com/webstatic/r/fb/fb-all-prod.pp.min.js
(gist copy (for posterity): https://gist.github.com/csmosx/55fae21b78468e6ba55f)
Which seems to be adding an FB global... thus overriding any existing FB global, including the Facebook SDK.
Why they would want to do this I have no idea, but the code is there.
Can anyone else confirm this behavior?
I was notified by Braintree Support on 2015-6-8 that this was fixed.
Braintree Case #400185