A data point that may help ...

I use AVG anti-virus which has a 'do not track' feature to discard tracking cookies. If I decide not to accept the 'Facebook Social Plugins' cookie, I see the large grey/gray area with no 'F log in' button per the picture in post #1.

However, if I configure AVG to accept the tracking cookie the button reappears and the grey area is correctly sized.

hth