Hello,
I've had this same issue before. So I applied the fix in here: http://www.askvg.com/fix-bold-blurry-or-hard-to-read-font-problem-in-windows-8-1/ and now it doesn't reproduce anymore! :D
Basically, you need to check the "Let me choose one scaling level for all my displays" checkbox in "Screen resolution" context menu, "Make text and other items larger or smaller"
LE: Hmm, it seems on facebook it still reproduces :(
Cheers.
I think this a brand new blur problem (!) MS managed to introduce with 8.1. It seems different than the grey-scale anti-aliasing on 8, IE and Office 2013. In any case, this new DPI problem finally made me uninstall 8 on all my PC's and go back to the brilliant Windows 7.