change/facebook-stub — comment on commit c9c70eff6c01ca0e63c8ef090ab8f7e515287581
File: src/facebook.js line 280
By herby:

There is nothing odd with this encoder. It does the correct Base64.
It is facebook who does things different (not only facebook, though). It is called Base64/URL, a variant of vanilla Base64 for safe use in URLs. This variant changes + and / characters into - and _ characters (so you miss one more replace).