Today, I received an email from Google Drive with title the "Changes to your Google storage plan". Its content is as below:
Huray! HTC One (M8) users' free Google Drive storage is now increased from 50GB to 100GB! This includes existing users who have already activated the free Google Drive storage and had already received 50GB free storage, now it is doubled up to 100GB too.
This 100GB of Google Drive storage is on top of the existing free storage of 15GB, which is shared among Gmail, Google Drive and Google+ photos.
Thanks to HTC and Google!