Sending XMLHttpRequest with FormData

You're missing the Content-Type header to specify that your data is form-like encoded.

This will work:

var xhr = new XMLHttpRequest(),
    data = "data%5Btumblelog%5D=drunknight&data%5Bsource%5D=FOLLOW_SOURCE_REBLOG";

xhr.open('POST','https://www.somesite.com/page', false);

// LINE ADDED
xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");

xhr.send(data);

EDIT

FormData is generally used to send binary data and will automatically set the Content-Type header to multipart/form-data (see FormData Spec and FormData Examples). However you have to make sure the server also accepts request using this MIME-type, which apparently is not your case, as you already tried and it didn't work.